Two quick things because I only have a moment to respond: 1. Yes, all of the commanders in the US Military have already come out publicly against Trump's statements, and have said - unequivocally - that they would not obey these orders. 2. Impeachment is only a black mark on the President's record. Impeachment does not equal "removal from office." Several Presidents have been impeached, and then go back to work in the Oval Office the next day. Removing a sitting President has never been done, and I am not sure off-hand how that process works. (Nixon left of his own accord.)
Removing people from office is what impeachment is for. "The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors." No president has ever been removed, but people have been removed from other offices.