Transferring from military of one country to another

Suppose I’m in a branch of the US military. I got married to a Belgian a few years back, am fluent in Flemish, and can now obtain citizenship in Belgium. How easy would it be to join the Belgian military? Would I have to go through boot camp again? And how about the reverse, a Belgian soldier or what-have-you joining the US military?

My best friend was an NCO in the Canadian Armed Forces, but happened to be a citizen of both the USA and Canada.

He had been in the computer industry for a few years when a contact in the United States Army started seriously recruiting him to join up. Their offer was an immediate captaincy, no boot camp or any of that crap, based on the fact that he was an experienced soldier.

He turned it down because it would have been about a fifty percent pay cut, but it’s apparently possible.