DanSC DanSC:
Two questions:
1. Who would be invading Canada?
Let's see, you've got the second largest country on the planet and it's virtually polluted with valuable resources and (absent a US alliance) it's all barely defended. You're kind of like the richest house on the block and everyone knows you've got money and jewelry just laying around the house and it's all protected by the international equivalent of:

Remove the USA from the equation and your question is better rephrased as,
who wouldn't want to invade Canada? DanSC DanSC:
2. Why would the USA tolerate having these invaders as neighbors?
Lack of money. Lack of interest. Lack of loyalty to our allies. We could be busy with a civil war. We could be busy with a war in or with Mexico. Etc.
A civil war in the US could provide some foreign powers an opportnity for adeventures in Canada and elsewhere in the Americas just the same as the last US civil war opened the door to a French invasion of Mexico.