Twenty-three miles from base, his local guide refused to go any farther. Bystanders warned of a large enemy force waiting up ahead, he reported. Captain Seth Thornton assumed the man was a double agent spreading Mexican disinformation, and ignored him.

If his superiors had wanted someone stubborn for the job, Thornton was their man. Eight years earlier, he had been on a steamship that went down off Cape Hatteras. He was rescued after being adrift in the Atlantic for six days, clinging to a chicken coop, while some other castaways didn’t last that long. But if the brass had wanted someone lucky, they should have kept looking. The guide was indeed a double agent, but happened to be telling the truth. Thornton pressed on another three miles into an ambush, and of the 55 dragoons in his patrol, 11 were killed and the rest captured, mostly after being trapped against the river. Thornton, separated from the others, was captured trying to get back to base.

It was April 25, 1846, on the north bank of the Rio Grande, somewhere northwest of the Mexican city of Matamoros. Thornton was scouting for an outnumbered U.S. Army expedition tasked with asserting the U.S. claim that the Rio Grande had become the U.S.-Mexico border.

His bad luck would be the pretext for a war that would change North America, but that was after the war took on a life of its own. The forgotten war Thornton thought he was fighting was far more limited.

April 25, 1846. Capt. Seth Thornton and his 55 U.S. Army dragoons struggle to fend off Gen. Anastasio Torrejón's 1,600 Mexican soldires after being ambushed on the north bank of the Rio Grande River somewhere northwest of the Mexican city of Matamoros. This incident was the first fight of the Mexican-American war.
April 25, 1846. Capt. Seth Thornton and his 55 U.S. Army dragoons struggle to fend off Gen. Anastasio Torrejón’s 1,600 Mexican soldiers after being ambushed on the north bank of the Rio Grande River somewhere northwest of the Mexican city of Matamoros. This incident was the first fight of the Mexican-American war.

The Texas Inheritance

The Mexican government had refused to recognize the secession of Texas in 1836. When the U.S. (despite Mexican outrage) agreed to annex Texas in 1845, it inherited Texas’ claim that its southern border was the Rio Grande. The Mexican government insisted the border was the Rio Nueces, 200 miles to the north. The intervening Trans-Nueces was an uninhibited wasteland of interest to neither government, but the Rio Grande stretched all the way to the Rocky Mountains, and using it as a border could turn Texas into an empire.

The U.S. legislation admitting Texas did not define its territory. Provisional recognition offered by Mexico at the same time (if Texas stayed out of the U.S.) didn’t either. Diplomats, not having done their homework, meant soldiers would die.

Gen. Zachary Taylor commanded the U.S. Army of Occupation that President Polk sent to the disputed Trans-Nueces territory after the annexation of Texas. Taylor became a national hero following his victories at Palo Alto, Resaca de la Palma and Monterrey.
Gen. Zachary Taylor commanded the U.S. Army of Occupation that President Polk sent to the disputed Trans-Nueces territory after the annexation of Texas. Taylor became a national hero following his victories at Palo Alto, Resaca de la Palma, and Monterrey.

Events developed slowly. After Mexico broke off relations, U.S. President James Polk sent Gen. Zachary Taylor (later president of the U.S.) in August 1845 to encamp an Army of Occupation in what is now Corpus Christi, just inside the disputed territory. With later reinforcements, Taylor had almost 4,000 soldiers, all regulars.

Meanwhile, in December 1845, Polk sent an emissary to the latest president of Mexico, Gen. Jose Herrera – who had indicated a willingness to talk – offering to cover claims by U.S. citizens who had suffered expropriations by Mexican officials and military units if Mexico would sign over the Trans-Nueces. (These claims amounted to about $3 million and had been a source of increasing tension.) He was offering up to $25 million for the rest of what was then northern Mexico.

Total revenue for the debt-mired Mexican government had been $20.6 million in 1844, its best year to that time, versus expenses of $31.3 million. Surely the money would be irresistible.

At the time, the U.S. dollar and the Mexican peso were equivalent.

As Polk’s emissary arrived in Mexico, however, his plans were leaked, and the Mexican press went ballistic. Who put up a for-sale sign? Herrera did not dare receive him. Herrera, to boost his own credibility, then ordered Gen. Mariano Paredes in San Luis Potosi to march north to the border. Paredes had recently helped Herrera overthrow Gen. Antonio Lopez de Santa Anna and send him into exile. Paredes had 8,000 soldiers – enough to stop, if not crush, Taylor. Some sources say 5,000, which was still enough.

Gen. Mariano Paredes used the 8,000 troops under his command to seize control of the Mexican government before turning his attention to Taylor's force in the disputed Trans-Nueces region.
Gen. Mariano Paredes used the 8,000 troops under his command to seize control of the Mexican government before turning his attention to Taylor’s force in the disputed Trans-Nueces region.

He marched south to Mexico City, where he installed himself as president.

With dollar diplomacy having backfired, Polk told Taylor to proceed to the Rio Grande. Taylor sortied on March 8, 1846. On March 28, the force arrived at the river opposite Matamoros after making a side trip to seize the port at Point Isabel, 23 miles to the northeast. Outside Point Isabel, they found no inhabitants until they were in sight of the river.

They camped on a duly leased farm field directly across the river from Matamoros and commenced an earthen fort that communicated permanence. It had walls 9.5 feet high and 15 feet thick, fronted by a ditch 8.5 feet deep. It had a vague star shape with six corner bastions, and a perimeter of 800 yards. Thousands of cubic yards of dirt had to be excavated with hand tools, and work continued night and day.

Matamoros had a garrison of about 3,000 soldiers, but there was no shooting – except at bushwhackers and deserters. The U.S. side wanted to finish the fort, and there was a command vacuum on the Mexican side.

Political Intermission

After his takeover, Paredes had dismissed the commander of the Matamoros area, Gen. Mariano Arista, a future president of Mexico, for not supporting his coup. He then had Gen. Pedro de Ampudia scrape together about 2,200 second-line troops left in San Luis Potosi and head north. His arrival in Matamoros brought the garrison to 5,200 soldiers and 26 cannons, but his appointment triggered a political crisis, as Ampudia’s reputation for brutality caused the locals to demand Arista’s restoration.

Paredes eventually complied, but then nothing happened while the seething Ampudia awaited Arista’s return. Meanwhile, the area was so restive that the Mexican commanders dared not use the usual IOUs or expropriations to gather supplies. Being broke, they faced chronic shortages.

Gen. Mariano Arista, a future president of Mexico, commanded the Mexican force operating in the Matamoros area. He and his army crossed the Rio Grande River in April 1846 hoping to catch Taylor's army on his return from Point Isabel on the Gulf of Mexico coast.
Gen. Mariano Arista, a future president of Mexico, commanded the Mexican force operating in the Matamoros area. He and his army crossed the Rio Grande River in April 1846 hoping to catch Taylor’s army on his return from Point Isabel on the Gulf of Mexico coast.

When he returned, Arista decided to act immediately while he still had a numerical superiority, surmising correctly that the outnumbered Taylor would ask for reinforcements. So after almost a month’s delay, operations resumed on April 23 when Arista sent a cavalry unit with about 1,000 troopers across the river upstream from Matamoros to get between Taylor and Point Isabel.

Hearing of movement, Taylor sent patrols upstream and downstream on the 24th. The next day Thornton (leading the upstream patrol) was ambushed. Taylor learned of it on the 26th when two wounded dragoons were sent back across the river.

Before the cavalry could do much more damage, Arista sent them to a point 13 miles downstream from Matamoros to cover the planned crossing of his infantry, which began April 30.

Taylor learned of the crossing the next afternoon and in two hours had his army on the road back to Point Isabel, hoping to secure it and then return to the fort with more supplies. At the hastily finished fort, he left an infantry regiment and three artillery batteries, totaling about 500 soldiers. He reached Point Isabel about noon the next day, May 2. At about the same time, Arista finished crossing the river and set out toward Point Isabel, leaving Ampudia to watch the fort.

Arista reached the Palo Alto area on May 3 and found that he had missed Taylor. He decided to block Taylor’s return, and camped his army at a place where he could cover the Point Isabel road, plus another “fair weather road,” to the east.

The bombardment of the fort also began that day, continuing during daylight for the next six days. Taylor’s force at Point Isabel could clearly hear the guns, adding urgency to their task.

By the afternoon of May 7, Taylor’s soldiers had fortified the port and filled 270 supply wagons, and the force headed back to the fort. They went seven miles before camping.

Palo Alto

May 8, 1846, U.S. forces under Gen. Zachary Taylor use their "flying artillery" to devastating effect against the Mexican line at the Battle of Palo Alto. Painting "The Battle of Palo Alto" by Carl Nebel.
May 8, 1846, U.S. forces under Gen. Zachary Taylor use their “flying artillery” to devastating effect against the Mexican line at the Battle of Palo Alto. “The Battle of Palo Alto” painting by Carl Nebel.

About noon the next day (May 8) they arrived at Tall Timber, the watering hole near Palo Alto named for the first trees encountered when coming from the coast. The flat prairie offered no cover, while thick knee-high cord grass, scattered ponds, and muddy ground retarded off-road movement. Random clumps of brush offered minimal concealment, but denser thickets did limit visibility to the west and south.

Gen. Pedro de Ampudia, who had commanded the Mexican Army of the North prior to Arista, now served as Arista's subordinate. Ampudia conducted the seige of Fort Brown on the Rio Grande and was a critic of Arista's tactics following the Mexican defeats at Palo Alto and Resaca de la Palma.
Gen. Pedro de Ampudia, who had commanded the Mexican Army of the North prior to Arista, now served as Arista’s subordinate. Ampudia conducted the siege of Fort Brown on the Rio Grande and was a critic of Arista’s tactics following the Mexican defeats at Palo Alto and Resaca de la Palma.

They found Arista’s force deploying across the road ahead of them, having marched from their campground to the southeast. Arista’s force was joined by Ampudia’s force coming north on the Matamoros road, having left the siege of the U.S. fort.

The U.S. force had about 2,200 soldiers on a line about 1,000 yards long, with the wagon train gathered in the rear. There were two 18-pounders in the middle of the line and two batteries of four six-pounders on each flank. The six-pounders were “flying artillery” trained by Maj. Samuel Ringgold. Following tightly choreographed drills, they would dash up to a point near the enemy line, fire several shots, and then dash away. They proved accurate enough to snipe at individuals.

Arista’s force was on a line about a mile long, with most of the cavalry on the west. He had about 3,700 soldiers. Two eight-pounders and six four-pounders were scattered along the line.

The Mexican artillery began firing about 2:30 pm as the U.S. force moved to within 700 yards. It was largely ineffectual, the balls routinely avoided as they bounced along the ground. The U.S. artillery responded, reaching the enemy without bouncing — and the results were ghastly.

That was the theme for the rest of the day, as the U.S. artillery dominated the field and kept Arista from using his numerical advantage. He tried to send cavalry around the west flank, but they literally bogged down and were driven off. Operations halted between about 4 and 5 p.m. due to grass fires. Another advance around the west was again driven off, but Maj. Ringgold was mortally wounded.

Arista then attacked unsuccessfully on the east flank, and his retreating soldiers disrupted the Mexican line. With the sun setting, Arista decided to pull behind a chaparral thicket to the south, and the firing ended.

The U.S. artillery had fired about 3,000 rounds. The Arista’s had fired about 600. There were six U.S. casualties and 40 wounded – about 2% of the U.S. force. Mexican casualties were probably 10 to 15%. The Mexican soldiers had generally behaved like they were on parade, yelling “Viva!” and closing ranks when (as witnesses put it) shots tore “lanes” and “vistas” through them.

Resaca de la Palma

Hand-colored Currier lithograph depicting the charge of Capt. Charles May at the Battle of Resaca de la Palma on May 9th, 1846 during the Mexican-American War. Image date: ca. 1846.
Hand-colored Currier lithograph depicting the charge of Capt. Charles May at the Battle of Resaca de la Palma on May 9th, 1846, during the Mexican-American War. Image date: ca. 1846.

During the night, Arista decided to put some distance between his army and the U.S force, so in the morning (May 9), he led it south on the Matamoros road six miles to its crossing of Resaca de la Palma, the terrain outside the road corridor becoming an increasing dense chaparral thicket. Arista deployed his forces behind both banks of the dry resaca, where they had cover from the U.S artillery. (See “Resaca Terrain” sidebar.)

His line extended about 1,000 yards on either side of the road, with seven cannons covering the crossing. He assumed that by the time Taylor’s force arrived and deployed, it would be too late to fight that day. Meanwhile, the bombardment of the fort, three miles to the south, could continue.

Taylor spent several hours fortifying his wagon train, and then followed Arista, making contact about 2 p.m. He immediately attacked while deploying arriving units to the right and left. Lt. Charles May won fame (that’s him in the fourth stanza of “Maryland, My Maryland”) by leading a mounted charge against Arista’s artillery, and there was back-and-forth fighting at the crossing.

A modern day view of the Resaca de la Palma. Photo courtesy of Armchair General.
A modern-day view of the Resaca de la Palma battle fields. Photo courtesy of Armchair General.

The units that deployed into the thicket to the left (east) of the road mostly encountered more thicket. Those on the right (west) found that the road closely paralleled the resaca, and they were soon in contact with its defenders. Small-unit melees broke out. Later arrivals found a cow path that led them across the resaca beyond the Mexican west flank. They pressed their advantage, and the entire Mexican position soon collapsed. Arista’s force withdrew in disorder across the river.

U.S. casualties were 45 killed and 98 wounded. Mexican casualties were listed as 154 killed, 205 wounded, and 156 missing.

Upon linking up with the fort that evening, Taylor found that its commander, Maj. Jacob Brown, had died earlier that day after being hit by a shell May 6, and there had been only one other fatality. Taylor named the earthwork Fort Brown.

U.S. reinforcements and heavy rain arrived in the next days. On May 17, Arista abandoned Matamoros to retreat about 200 miles to Linares, leaving about 400 wounded. He started the hastily organized march with about 4,000 soldiers and arrived 11 days later with only 2,638.

The After-War

U.S. President James K. Polk had his eyes on the Pacific Ocean and "Manifest Destingy" when he offered to buy the Trans-Nueces region and the rest of northern Mexico after the 1845 annexation of Texas. He eventually acquired the land by force of arms following the Mexican-American War.
U.S. President James K. Polk had his eyes on the Pacific Ocean and “Manifest Destiny” when he offered to buy the Trans-Nueces region and the rest of northern Mexico after the 1845 annexation of Texas. He eventually acquired the land by force of arms following the Mexican-American War.
U.S. President James K. Polk had his eyes on the Pacific Ocean and "Manifest Destingy" when he offered to buy the Trans-Nueces region and the rest of northern Mexico after the 1845 annexation of Texas. He eventually acquired the land by force of arms following the Mexican-American War.
U.S. President James K. Polk had his eyes on the Pacific Ocean and “Manifest Destiny” when he offered to buy the Trans-Nueces region and the rest of northern Mexico after the 1845 annexation of Texas. He eventually acquired the land by force of arms following the Mexican-American War.

On May 11, two days after Resaca de la Palma, Polk received word of Thornton’s ambush and asked Congress to declare war, since the Mexican Army had “shed American blood upon the American soil.” Congress complied, and authorized the expansion of the U.S. Army by a factor of six.

Of course, the Trans-Nueces War was already settled, but not for the last time, the U.S. found that superior weapons, logistics, training, organization, and more had produced a lopsided victory but not an exit strategy. Taylor would press on to Monterrey and then Saltillo in northern Mexico, and other expeditions would seize all the territory that Polk had offered to buy — and the Mexican government remained unresponsive.

U.S. planners prepared an expedition (under Gen. Winfield Scott) to march overland from Vera Cruz to Mexico City, where they hoped to dictate terms. Questionably, they also let Santa Anna return to Mexico through the U.S. blockade.

The crowd-pleasing but erratic Santa Anna reasserted control in Mexico, and then by supreme effort concentrated an army of more than 20,000 soldiers against Taylor, whose best troops had been sent to Scott. After a 240-mile advance across a desert in winter, and then losing the battle of Buena Vista/Angostura on Feb. 23, 1847, despite a three-to-one advantage, followed by a retreat across the same desert, he’d lost half his army. He scratched together a force to oppose Scott’s advance, but between losing more battles, violating truces, and soliciting bribes from Scott, he eventually lost Mexico City and resigned.

Thornton (freed in a prisoner exchange) marched with Scott, and was killed by artillery fire while leading another reconnaissance on August 18, 1847 — his luck unchanged.

Santa Anna’s successors ratified the Treaty of Guadalupe Hidalgo on Feb. 2, 1848. It not only set the border at the Rio Grande but gave the U.S. what is today California, New Mexico, Nevada, and Utah, most of Arizona and Colorado, and slivers of Wyoming, Kansas, and Oklahoma. The U.S. gave Mexico $15 million and covered the pre-war claims.

At almost the same time, word of a gold strike began filtering out of California, and the Trans-Nueces was forgotten.

It remains forgotten. For instance, the Rio Grande Valley is the only metropolitan area in the contiguous U.S. not reached by the Interstate Highway System. Sadly, due to upstream irrigation, the river has about one-fifth the flow it had in 1846.

The Fort Brown earthwork was abandoned shortly after Taylor occupied Matamoros following Arista’s departure. Most of it was obliterated by later levee construction. The southwest corner survives as a serpentine brush-covered mound in the northwest corner of the Fort Brown Memorial Golf Course. It is actually outside the border fence, but that’s only appropriate. It shrugged off 2,700 rounds of Mexican artillery. It has nothing to fear from the south.

Fort Brown the military post was later established a few hundred yards to the north of the earthwork, and the city of Brownsville grew up beside it. Decommissioned in 1946, it is now a college campus.

At Resaca de la Palma, the spot where May won fame is now the bridge where Parades Line Road crosses the resaca, which has been dredged and flooded to serve the municipal water system. The battlefield northwest of the bridge, where most of the fighting took place, is lost under suburban development. A largely vacant 35-acre tract of land just northeast of the bridge, formerly a polo field, was finally acquired by the U.S. National Park Service in 2011.

A modern day view of the Palo Alto battlefields. Photo courtesy of Armchair General.
A modern-day view of the Palo Alto battlefields. Photo courtesy of Armchair General.

Aside from improved drainage, the Palo Alto battlefield has changed little. The Palo Alto Battlefield National Historical Park was established in 1978 and now owns about 1,600 acres of the battlefield, mostly in the area the Mexican army defended.

As for the Thornton ambush site, in late 1847, freshman Congressman Abraham Lincoln tried to force the government to prove that the site had actually been American soil. He was ignored—and that’s a pity, since proof would have involved identifying the site, which we can’t do today.

A memorial was erected in 1936 marking a candidate site on Highway 281 about two miles west of Los Indios, but it is almost a mile from the river (often marked by the rusty border wall) and cannot be correct.

Basically, Rio Grande has meandered considerably since 1846, and the ground the U.S. officially went to war with Mexico to defend has almost certainly reverted back to Mexico.

*A version of this story was originally published in the November 2014 issue of Armchair General magazine

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Lamont Wood

Lamont Wood is a San Antonio-based writer who does the family shopping.