Throughout the nineteenth century American expansion was aggressive and unbridled, causing fear throughout the Western Hemisphere. This was especially true for British North America, which had a history of dealing with American antagonism and repelling American invasion. By the time the American Civil War began in 1861, British North America had dealt with eighty-six years of American ill will. In the four years of the American Civil War, tensions between British North America and the United States increased to the point where the British colonists sought the reorganization of the provinces as a means of greater defense against aggression. In terms of how significantly the American threat affected British North American Confederation, as the historian Alfred Leroy Burt noted "it is more than doubtful...there could have been a Dominion of Canada then or for along time afterward, if at all, without the Civil War."
Roloff '03, Katrina L., "Confederation at the Tip of a Sword: How the Threat of American Aggression Created the Dominion of Canada" (2003). Honors Projects, History. 6.