Thoreau to write his essay Civil Disobedience, and a congressional freshman from Illinois named Abraham Lincoln to claim that it had been started by “the sheerest deception.”

A more serious dissension grew in the ranks. Out of nearly 41,000 Regular soldiers who served in Mexico, no fewer than 5331, or nearly 13 percent, deserted, a figure that had never been approached in the U.S. armed forces. What was going on? There were, of course, the usual privations of Army life: bad food, poor shelter, boredom, disease. But two additional factors loomed large: religion and ethnicity.

A substantial number of the American forces and of the deserters were immigrants born in Germany or, especially, Ireland. According to Peter F. Stevens, in his meticulously researched history The Rogue’s March: John Riley and the St. Patrick’s Battalion , out of some 5000 “Irishmen” in the ranks, nearly 1000 went over the hill, along with 445 Germans and 457 men born in other European countries.

Many of the Irish could barely have considered themselves Americans, having walked straight off the “coffin ships” from their famine-ravaged homeland and into the Army. They also walked straight into the rising nativist, anti-immigrant tide of the late 1840s. Citing numerous letters, memoirs, and court-martial records, Stevens shows that the deserters were bitterly resentful over what they saw as a double standard of military justice. European-born troops were routinely cursed as “foreigners” by nativist, “American” officers. They were also whipped, beaten, branded, chained, and subjected to such evocatively named punishments as “riding the horse,” the “barrel top,” and the “buck-and-gag” for the smallest infractions or mistakes in drill. Feelings between some of the officers and men deteriorated to the point where officers were attacked. Braxton Bragg, then a brilliant but particularly harsh commander of artillery, survived two separate attempts by his men to kill him by rolling a lit shell into his tent as he slept.

Hundreds of soldiers began to desert even before war was officially declared, diving into the Rio Grande and swimming over to the Mexican army in Matamoros. They were lured by a series of clever propaganda pamphlets that promised them better pay, higher ranks, acres of land, and the chance for American Catholic troops not to fight an unjust war against their fellow believers.

The religious appeal seems to have had a special resonance. At the time, the U.S. Army did not have a single Catholic chaplain. President Polk hastened to rectify this, but the desertions continued. Worse yet, an ambitious young Irishman named John Riley began to organize some 200 of his fellow deserters into a cohesive and deadly military unit of their own. It would be known as the St. Patrick’s Battalion, the San Patricios.

Riley remains to this day a shadowy figure, slipping in and out of history like some trickster archetype. Little is known about him, right down to the proper spelling of his last name. He may have

been a gunnery sergeant in the British army before emigrating to the United States; certainly, he was the most defiant and ambitious of all the deserters and a natural leader.

Before long, Mexico made him an officer, a position he would have found all but unobtainable in both his previous armies. As a skilled gunner, he was able to mitigate one of the Americans’ greatest advantages, their “flying batteries” of artillery, which shuttled swiftly around a battlefield, unlimbering their guns and getting off the first round in less than a minute. Again and again, at engagements from Monterrey to Churubusco, Riley’s command used these same tactics against its old comrades with devastating effect.

Their success delighted the Mexican dictator Santa Anna, who planned to gather some 3000 American deserters and, with them, dissolve Taylor's and Scott’s invaders. Who was to say that he wouldn’t succeed? It was one of those moments when history seemed to be completely fluid, when it seemed entirely possible that the more numerous, seasoned Mexican troops could win the war and that North America could remain divided north from south and possibly split permanently along ancient religious lines, instead of becoming the vast, polyglot nation that it is today.

But the Irish held. Bad as they were, the desertions never came close to halting Scott’s final advance. The San Patricios fought ferociously to the end, suspecting that capture meant the rope. They were right: 30 of them were hanged from a bluff overlooking the final battle of the war, the breathtaking Army and Marine assault on the towering stone fortress of Chapultepec.

The condemned men were stood on wagons with nooses around their necks and told that, when the U.S. troops had taken the fort, they would be hanged. From daybreak on September 13, 1847, they stood on their wagon beds, watching the assault through the dust and the broiling heat. At 9:30, the serpent-and-eagle banner of Mexico was struck and the American flag run up. Whether out of relief, out of admiration, or out of some last, wry defiance, the condemned men cheered. The wagon horses were promptly whipped up, and the men hanged.

John Riley was not among them. He had deserted before hostilities had been declared, and therefore, under the Articles of War, he was not guilty of a capital offense. This finding brought outrage from both American officers and men, but General Scott stuck by it. Rather than that the life of Riley should be taken, “he would rather with his whole army be put to the sword in the assault he was about to make upon the gates of the City of Mexico,” one of Scott’s staff officers wrote of him, providing a nice lesson in the rule of American law.

Riley was lashed and branded on both cheeks, but he was then released and became a brevet colonel in the Mexican army. Afterwards, he slipped back through the cracks of history, possibly back home to Ireland. In Mexico, each

year, two commemorations take place to honor the memory of Riley and his San Patricios.

A better memorial, I think, are the 86 Certificates of Merit that were awarded to Irish-American soldiers who remained loyal to their new country. The memory of their fidelity, in the face of intense bigotry, should serve as a guard against those who now seek to divide us from within.