Canada has long been one of the United States' closest allies—but the relationship between those nations hasn’t always been so friendly. One of the first campaigns of the American Revolution was an attempt to annex Canada, and off-and-on American hostilities were one of the key motivations behind Canada’s creation as a unified country.

Besides the American invasion of Canada in 1775, and continued fighting throughout the War of 1812, Canada has faced American invasion on several other occasions.

1. The Dickson Filibuster // 1836

The first minor American invasion of Canada was very minor indeed. James Dickson first appears in history in 1835, when he declared that he would create an Indian federation with himself as president. The first step in his plan to create an empire spanning the western half of North America was to seize the fur outposts of Manitoba, where Métis scouts had a tense relationship with the Hudson’s Bay Company. He left Buffalo, New York in August 1836 with 60 volunteers.

By the time they reached Manitoba in late December, most of Dickson’s volunteers had deserted and he only had a dozen followers. Hudson’s Bay Company officials offered jobs to his men, and a couple of months later Dickson gave up, vanishing into the West. He left no trace after disappearing in February 1837, although several of his volunteers became prominent local leaders.

2. The Patriot War // 1837–1838

Canada was divided into several colonies in 1837, and the large colonies of Upper and Lower Canada (basically modern Ontario and Quebec) were controlled by conservative cliques. When the Panic of 1837 devastated economies across the world, including Canada, desperation bred rebellion.

In August 1837, a militia called the Sons of Liberty formed with the intent of expelling Britain from Quebec and declaring independence, under the leadership of Louis-Joseph Papineau. Papineau’s men first fought the British on November 6; when they were forced to disband in mid-December, the fighting had taken 300 Canadian and British lives. Another rebellion in Upper Canada under William Lyon Mackenzie was also suppressed.

The leaders of what became known as the Patriot movement escaped to the United States, where they were wildly popular. They began recruiting American volunteers; on December 13, Mackenzie occupied a small Canadian island near Niagara Falls and declared himself President of the Republic of Canada. He gathered 500 Canadian and American volunteers. The commander of British forces across the river decided to prevent Mackenzie from gaining more reinforcements by stealing a steamship, the Caroline, anchored at Mackenzie’s base. Sixty Canadians crossed the river and killed an American watchman before scuttling the steamship.

The fury over the killing of an American on U.S. soil threatened war, and President Martin Van Buren dispatched a military unit under General Winfield Scott to defuse the crisis. The crisis boiled over again in November 1838, when 400 American filibusters crossed the Canadian border. A two-week British bombardment killed thirty of the invaders; another 160 were captured. A last attack near Windsor, Ontario in December ended with 23 dead.

The Patriot War spurred important British reforms, and led to talks which resolved all border disputes between the United States and Canada. However, the resolution of crises in North America could not protect Canada from invasion as long as England faced another crisis in Europe—Irish nationalism.

3. The Fenian Raids // 1866–1871

Ireland was first invaded by England in the 12th century. Many Irish fought and died to expel the English for centuries, but the Great Famine of the 1840s galvanized the independence movement. Irish nationalism was given a new spur when Irish emigrants to the United States found themselves a despised minority. By the time the American Civil War broke out, the Irish Revolutionary Brotherhood (commonly known as the Fenians) had agents across the United States.

One hundred and seventy thousand Irish immigrants fought in the Civil War, and many of them ended the war hardened soldiers. Ten thousand of them joined the Fenians after the war’s end in 1865, under the leadership of Brigadier General Thomas William Sweeny. They also elected a government in exile in New York and embraced Sweeny’s plan to conquer Canada and hold it hostage until Britain agreed to grant Ireland its freedom.

In April 1866, 400 Fenians gathered in Maine but disbanded under the watchful eyes of U.S. troops. No one in the federal government was enthusiastic about reining in the Fenians (and thus losing the Irish vote), and as a result, the Fenians were able to organize a much larger campaign. On June 1, the Fenians invaded Canada near Fort Erie; 1100 crossed before the U.S. military shut down the river crossings. After brief fighting that left nine Fenians and 15 Canadians dead, the Fenians retreated—about 850 were captured by U.S. authorities and another 100 were caught by the Canadian militia. Another raid a week later ended with no casualties, largely because federal authorities seized most of the Fenians’ arms and ammunition. The crackdown ended a few months later for political reasons, but the damage was done; the Fenians were virtually disbanded.

Ironically, while the 1866 raids accomplished little for Irish nationalism, they were a defining moment for Canadian nationalism; in February 1867, the various Canadian colonies were combined into the Dominion of Canada.

Some Fenian factions continued fighting; a shipful of Irish-American fighters briefly landed in western Ireland in May 1867. In 1870, a Fenian raid was ambushed at the border and surrendered to American authorities after they lost five dead. In October 1871, a few dozen Fenians marched for Manitoba, hoping to join forces with Metis Indian rebels—they were instead arrested by U.S. forces before they crossed the border. While they attempted a few more invasions of Canada (including building an early submarine), none of them ever really got past the planning stages—an anticlimactic end for a movement that had once counted thousands of men.

4. The Oka Crisis // 1990

The last American invasion of Canada was a curiosity—a group of armed Mohawk Indians who were U.S. citizens traveled to Quebec at the invitation of militants at the Kanesatake Mohawk reservation to protest the expansion of a golf course on disputed land; politics within the Mohawk nation were deeply divided on both sides of the border. On July 11, 1990, after two months of confrontation, over 100 Quebecois police charged the Mohawk militants’ barricades; one of the policemen was killed in the subsequent firefight.

By August, the 200 militants (including 30 U.S. citizens) were surrounded by 4400 Canadian Army troops. Many people were evacuated on August 28, although the standoff continued for another month and dozens of Mohawks and Canadian soldiers would be hospitalized for injuries sustained in fighting. The government ultimately bought the land to stop the development of the golf course.