Pontius,
World politics is a very complicated topic. The French came to our aid in 1775 or so and joined with us against the British. We all came together against the Germans twice. We then all joined together during the Cold War. Those were the "big" events....numerous "small" events have shown us to be only somewhat steadfast allies (including the French, Germans, British, Italians, etc.)
A soldier does not have the luxury of picking his foe...that is done by the Diplomats and politicians generally. Many a Marine has seen combat in support of US Big Business such as the Banana Wars. I am sure your forces have been thrown into wars around the world during the Empire days.
As to who I would pull the trigger on in your question is simple. Whoever my Commander-In-Chief told me to is the answer. I might find it repulsive, I might find it against my desires. However, if the Big Boss let me and my brothers off the leash....I would do my duty as I swore to do.
Put yourself in the shoes of the American forces that landed in North Africa....to be fired upon by the French and take casualties and then in turn land on the shores of Normandy to liberate the very same people from the German Occupation. How did they feel about that bit of irony? The Germans sure put up a much more aggessive defense than did the French thus the cost was much higher.
The decision as to who is the "enemy" comes from on high. Those of us in the trenches have to believe the cause is just and do their bidding in as honorable a way as we can.
Just this week, I learned Henry Kissinger, while Secretary of State, told the Chinese in secret meetings the United States Government would have accepted a Communist South Vietnam if it happened a decent interval after we withdrew. Over 58,000 Americans died in that war....and the government had given up long before we pulled out. How many of my friends died after that conversation I wonder?
Do I believe everything I hear from my political leaders.....no. Do I have unfailing trust in my senior commanders from those days....NO!
Is it right to be critical of those who will send us off to fight and die.....absolutely. But...we have to do so in an honourable way. Once we get stuck in one of these wars, we should all focus upon "winning" and not do anything that undermines the troops that are fighting the war.
I fear this generation of war fighters will be defeated not by the enemy but by our own side. After serving in Vietnam and knowing what that feels like...it would break my heart to see it happen again. Our troops deserve better of those who voted so quickly to send them off to war.