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During that century the two countries nearly went to war again at least three more times. As prime minister in the 1840s, Sir Robert Peel was acutely conscious of the American threat to Canada, and warned Parliament of possible war. This was the age of ‘manifest destiny’, which for many Americans meant their country’s destiny to rule the whole of North America, including Canada. At the 1844 presidential election James Polk and the Democrats campaigned on a bellicose platform, demanding the territory which would become the Canadian province of British Columbia, the Pacific coast from Oregon north to the 54th parallel and the border with Russian America, now Alaska: hence the unwieldy slogan ‘Fifty-four forty or fight!’ When Polk was elected, he backed off and found an easier target to the south, embarking on the Mexican–American War of 1846.
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Geoffrey Wheatcroft (Churchill's Shadow: The Life and Afterlife of Winston Churchill)