There are plenty of examples of smaller countries holding their own or even defeating countries much more powerful on paper. We experienced that in Vietnam, the Soviets did in Afghanistan. Wars are chaotic and often unpredictable.
More importantly, the U.S. has no desire or reason to attack Canada. It's not about ability. I doubt Trump seriously wants an invasion of our northern neighbor or to make them the 51st state. It's a negotiating tactic and it's working in the sense that has them rattled & they are making concessions.
“Incline my heart to your testimonies, and not to selfish gain!”
Psalm 119:36