Also, obviously something happened to get from US to Gilead. I just accepted that, during that time, Canada was able to shore up its defenses to resist that. To put my hypothetically thinking cap on, my guess is that the leadership in Gilead cut off a lot of the US-allies (sorry, I can't see many 1st world nations sticking with a nation that would do what Gilead would do). Canada, as part of the British Commonwealth, would be afforded some protection by the British government and military. As it is now in the show, Gilead is barely holding it together. Trying to take on Canada, with British backing, would just be too much.