Canada

Here is the thing. Ukraine has, or could easily within weeks have, nuclear capability (they were actually instrumental in developing Russia's nuclear capability, and they still have the infrastructure, knowledge, and skills) and that is actually to their detriment. It only promotes a 'first strike' mentality in the Russians.

The land border between Canada and the US is just too long for either side to defend.

But here is the thing. The only way the American army could have the resources to do it would be through the National Guard. American forces are spread too thin throughout the world. The National Guard is under the control of the individual state.

The biggest states would be well prepared to sign non-aggression treaties with Canada, meaning their National Guard could not be used. The individual States can not directly sign a treaty, but they CAN prevent their National Guard from participating. If Trump tried to take charge of them, it would for sure lead to a Constitutional Crisis. Since the Commander-in-Chief of the National Guard Units is the governor of each state, not the President, the troops are only answerable to the chain of command that ends at the state level. The individual states would have all the reason they need to secede. It would be civil war second inning in America. And some National Guard units have their own fighter jets and air bases.

That chaos is Canada's best bet for defense.