Between 1792 and 1812, about 30,000 Americans left the republic to seek land in Upper Canada. They became known as the “Late Loyalists,” but most were just looking for the cheap land that the British granted to new settlers who would take an oath of allegiance. By 1812, these Americans and their children were most of the inhabitants in Upper Canada (now Ontario).

That fact led American leaders to target Upper Canada for invasion—in the hope that the settlers would welcome and assist the invaders. But the British expected those settlers to defend the colony against any American invasion. When war came, these newcomers faced a tough choice: would they fight for their farms and against their kin and former neighbors in the invading armies?At the same time, thousands of British emigrants poured into American seaports.

Primarily from restive Ireland, the newcomers fled from British rule to seek economic opportunity and political liberty in the republic. Becoming staunch Republicans, the Irish-Americans sought a revenge on the empire. They served in disproportionate numbers in the armies that invaded Upper Canada. Defeats cast scores of Irish-Americans into British prisons, where they had to enlist in the royal forces or face trial as traitors. The British took that hard line because they feared for the loyalty of their own soldiers in Canada—who were mostly from Ireland.

By punishing the Irish captured bearing arms for the United States, the British set an example meant to preserve discipline in their own heavily Irish army along the border. To save the imprisoned Irish-American soldiers, the American government threatened to execute a captured Briton for every American immigrant hanged by the British. In an escalating spiral, most of the prisoners on both sides became hostages for the fate of the Irish-American soldiers threatened with British trial.

So this civil war pitted the Irish against the Irish, Americans against other Americans, and native peoples against their own kin.The War of 1812 began as a second round of the revolution but it ended as a transition into a new era where the United States and the British had to coexist, however uneasily, along a more sharply defined boundary. Despite later boundary controversies and even cross-border raids by private adventurers, the two nations avoided another war. Paradoxically, by producing a military stalemate, the War of 1812 reassured both sides that they could survive the presence of the other in a shared continent. Canada and the United States took shape from the bitter experiences of a civil war in a shared borderland.