Canada should be prepared for one of the following (and it can do so by having nukes to deter invasion)
1) The US itself decides to invade Canada for its resources and land
2) Someone else decides to invade Canada, the US is politically unwilling to fight on behalf of Canada (this is the US does allow someone to invade scenario)
3) The US collapses as a result of political and economic turmoil/instability or moves into civil war... Canada can get invaded by anyone at this point




