The answer is probably not in any country as long as those two countries were not at war with each other. A head of state has diplomatic immunity. Even suing a head of state for actions taken while he was head of state is generally not allowed.
Also no war crimes were technically committed by the US in the Iraq war. Of course there are some who may argue this.
While war itself is against international law, there are exceptions to it.
We can legally go to war in self-defense or if the UN security council votes 9 out of 15 countries with no vetoes by China, US, Russia, France, or Great Britain. Basically this means that if we were determined to go to war with a country, we could almost always find a legal excuse for it. The self-defense exception has very ancient roots and has even justified foreign takeover of a country in the ancient world. In the modern world, a permanent takeover of a foreign country is almost impossible to justify though except through UN sanction or the self-defense exceptions, and both arguments would meet with extreme resistance (even with the invading country's citizenry).
Unfortunately international law is hard, if not impossible to enforce in a foreign country. Basically common sense rules and then international law is used to justify it in one way or another for the most part. However a head of state in any country is pretty much above foreign law for the most part, both criminal and civil. The best a foreign power could hope to do is deport that head of state and/or file a protest.
However a war with Canada is a scary thought. It's not as if the US would lose, but it would open the door to a slew of problems for both sides. The US/Canadian border is the largest unprotected border in the world. Policing such a border effectively would be close to impossible.
The bottom line is that the US doesn't want to go to war with Canada and would try to avoid it. I would presume Canada would have even less of an interest in going to war with the US. Therefore something like this wouldn't happen easily.