Would the officers accept the order and pass it down to the troops, or would they refuse and/or resign their commissions in protest?

Some would refuse and resign (that's actually already happened in regard to other issues, by the way), but there would invariably be idiots who would follow those illegal orders. I mean, Pete Hegseth isn't remotely the only unqualified moron in uniform.

There is no rational reason for anyone in America to demand to be given the sovereign lands of one of our NATO allies, particularly the so-called President of the United States sworn to uphold the Constitution. It's off-the-charts insanity and abuse of power that would violate the Constitution as well as both domestic and international law; however, there isn't a president on either side of the political aisle who hasn't violated international law in some form or fashion.

Would the troops follow such an order, or would they refuse to fight?

"I was just following orders," definitely comes to mind. Some of them would obviously easily recognize the illegal orders, but I promise you they do allow inveterate morons into the United States military in every country on the planet Earth. The third-world and "shithole countries" (and aspiring ones) actually elect/appoint morons to lead because "stupid is as stupid does."

It would definitely take a special kind of stupid to attack a NATO ally so, yes, President Pedophile Protector definitely qualifies, and his appointed idiots would follow, and many of the enlisted morons actually elected Trump so... connect the dots.

The Guardians of Pedophiles and their Castrated Caucus will have to stop "His Ignorance" the wannabe king. Having said that, His Ignorance definitely blows a lot of hot air and then TACO.