1) Many Americans believed the invasion would be a cakewalk, particularly since Britain was so distracted by the Napoleonic Wars in Europe.
2) War Plan Red first set out a description of Canada's geography, military resources, and transportation, and went on to evaluate a series of possible pre-emptive American campaigns to invade Canada in several areas and occupy key ports and railways before British troops could provide reinforcement to the Canadians
3) United States would be less likely to attack Canada if it were a self-governing country rather than separate colonies of Britain. The fear of the United States helped to strengthen the call for Confederation.