And the Commander-in-Chief defines the current 'enemy'.  Obama says he can kill anyone  in Yemen or Libya or Syria or Pakistan or Afghanistan or anywhere in the world based on the Bush-authored Congressional approval voted for by Hillary Clinton.

I prefer a Congressional Declaration of War, but Obama has set the standard for the last eight years, so I guess we'll see whether Russia is declared an enemy by President Trump, or not.  

The part of a military oath  that applies there is "that I will obey the orders of the President of the United States".  I did that under several Presidents, starting with JFK during the Cuban Missile Crisis.  Now Obama says Cuba is no longer an enemy.  A military person obeys the "orders of the President of the United States", therefore today Cuba is not an enemy.  

The civilians define the 'enemy';  the military goes out to die or be maimed to effectuate and implement that definition.  

Simple, right?  Even an armchair warrior can figure it out.