The military mind is an odd mix of duty and conscience. In the majority of situations, the duty will kick in and without doubt they will run to the sound of guns. Length of service, amount of satisfaction they have with their environment, compensations when off-duty, and the support of family--either accompanying or remote, will determine the outcome of their course of action.
If I were still in the saddle, I would definitely be in the "no way" corner. My security clearances were the highest in the land, and I would do nothing to jeopardize them. Having said that, it would take a force bigger than just my immediate superior to get me to betray my oath to the Constitution and NOT to the Felon in Chief.
As you postulate, MJ, should the order come down to fight FOR Russia (not the bad, old USSR) against the beleaguered Ukraine, I would have to decline. Turkey has the right idea. Go in, finish the conflict, and get the Hell out. We should have done that in Vietnam, and probably would have had not Johnson sent those two destroyers (I was ON one of them) into the Gulf of Tonkin and forced North Vietnam to retaliate. This was his aim, to force Congress to allocate more men and equipment as targets. But I digress.
Russia is, by far, even more dangerous than the USSR was. They now have only loose control of all their firepower. They've lost their satellite countries, and are now desperate. Putin viewed Felon47 as an ally his first term, working to get him elected. Then, the second round was orchestrated by another oligarch who wants nothing more than to tear our government apart in order to make Felon47 even stronger.
The effects are even now being felt down at my level. Every contract doctor/nurse at the base hospital has been terminated. They're kicking every one of us who have been accepting care from the base (In my case for 32 years) out into the cold world of Medicare, which is itself under bombardment from Felon47.
Sorry. I rambled a bit, but to summarize: No way would I fight FOR Russia. I would fight as a common guerrilla FOR the Ukraine before I'd do that.
Bill