Can the US military refuse an order to invade Greenland and Panama?

If Trump uses the War Powers Act to order the military to invade Panama and Greenland he will be committing a war crime under international law.

An American officer AFAIK is not required to follow an illegal order (a precedence established at Nuremburg - "We were only following orders" is not an excuse under international law).

Could the American military therefore refuse to carry out such an invasion if ordered by the commander in chief?