So, I just started playing Risk and I try going for North America, it typically works out rather well. Lately, after my second game, my opponents seemed bent on trying to deny me North America, sometimes they'd claim a territory and when placing additional armies, at the start of the game, they'd amass 3 there and one other player would also do this. They managed to stall me long enough that I could not get the continent in a timely manner and they managed to get their continent bonuses quickly, which lost me that game.
I'm going to assume this is going to likely be a common thing I run into. Are there certain territories in North America I should be going for? I always grab Central America and the Western US first. After that, I try to claim Alaska and Greenland.