In WWII Canada had the world's fourth largest air force, fifth largest navy. Canada was involved in WWII more than two years before the United States joined.

You keep the military for the times, or at least most countries outside of the United States and its absolutely outrageous military spend does. If you think the US went to Iraq or Afghanistan or Vietnam for Canada (or countless exercises throughout the rest of the world, usually to prop up whatever the favored dictator was, and to promote US interests), or that Canada owes US taxpayers for that, you're profoundly mistaken (though Canada went heavily into Afghanistan for the United States, the single time NATO was actually invoked for defense).

The US exports variants of a lot of military hardware for sure, but it still keeps the cream of the crop tucked away, such as the F-22.

Don't think for a second the US military hasn't planned for having to wage war against a country that it's outfitted with it's own technology. Many exported items have reduced capabilities, and lack the communications systems that allow information sharing between all military assets. Information at the front line is what wins battles.

Most of Europe, and most of Asia too.