I could certainly see this as justification for invasion even though (despite much misinformation from both Republican and Democratic administrations) none of the 9/11 attackers arrived in America through Canada. In fact, Canada (in cooperation with American authorities) foiled not one but 2 significant terrorist plots in the weeks around 1 Jan 2000.

I'm not positive, but I think the last guy to enter the US from Canada to successfully commit a terrorist act may have been John Wilkes Booth who was heard bragging about assassinating Lincoln 3 weeks prior to carrying it out in a Pub in Montreal. But that doesn't really count because Booth was an American and was really a domestic terrorist.