Remember when former Canadian Prime Minister Justin Trudeau told President Trump the Canadian government could not continue its economic policy if the United States demanded they become self-sufficient.
One of the key reasons cited by Trudeau was the Canadian dependence on the USA for defense. Canada simply could not spend the amount of money needed to create their own military at a level required for independent defense and simultaneously follow their govt spending plan. Canada relied on the U.S. military as their defense strategy.