In 1837, a U.S. steamship, the Caroline, carried men and supplies to a site on the Niagara River a short distance above the falls. Their purpose was to render assistance to William Lyon Mackenzie, who was leading a rebellion in Upper Canada. Many in the United States were sympathetic to the rebels, seeing them as the successors to the American independence movement.
Late in December, Canadian loyalists crossed into American territory, cut the Caroline loose and allowed it to drift over Niagara Falls. One U.S. citizen was killed in the incident and American outrage quickly followed. In retaliation, the British steamship Robert Peel was attacked and burned and several small raiding parties went into Canada.
General Winfield Scott was dispatched to the scene by President Van Buren, primarily for the purpose of restraining further American incursions into Canada.
In November, 1840, New York officers arrested a deputy sheriff from Upper Canada (Ontario) named Alexander McLeod and charged him with murder and arson in connection with the raid on the Caroline. The British foreign secretary Lord Palmerston objected that the trial was illegal and that any action McLeod might have taken would have been a legitimate defense of Canada. The U.S. authorities proceeded with the trial anyway, and McLeod was acquitted in October 1841.
The 1842, Daniel Webster and Britain's Lord Ashburton negotiated the Webster-Ashburton Treaty. In the course of the discussions, Ashburton suggested that perhaps the British should have apologized for attacking the Caroline. For his part, Webster admitted that perhaps the raid had been in self-defense. The British were pleased to see that no indemnification was being asked, and Webster chose to interpret the British comments as an apology. The case was closed.