The US has the most powerful, technologically capable military in the world.
There is no conceivable way, even if Trudeau conscripted every man and woman over the age of 19+ that we would have the capability of holding the 49th parallel.
It's the largest undefended border because it's impossible to defend 3000+ miles of territory.
We are completely dependent on the US' goodwill and respect of our borders which under Trump would be non-existent.