If you are speaking of WWII, the US only entered the war formally after the

attack on the US Navy at Pearl Harbor. Not sure why the US didn't get more involved after the invasion of the

Phillipines but I'm not fully up on that part of history.
The US at the time had a policy of non-involvement

(military) otherwise. There was economic support of Britain and other countries before the formal declaration of

war.





Economic support of Britain included sending ships there and since the

Nazis tried to sink them, of course, the ships had to be guarded and it meant a direct war against German.






If I recall the US only got involved in the European theater due

to the military/political link between Japan and Germany.




Well, I don’t think that Germany was that much involved in war in Asia

to consider it. America had to decide whether it supported England (and it meant a direct war against Germany) or

not. Yes, I am sure that many citizens of the US would prefer not to be involved, but the politicians knew that

there was no choice.




Politics is strange... Even though the US was "friends" with

France for example at that time, we did not get militarily involved with the war even after France's colonies in

Asia (eg Veitnam) were invaded by the Japanese. During/after WWII we got roped into some sort of treaty with France

about coming to their aid if they or one of their possessions was invaded which is how we got mixed up with the

communist "invasion" in Veitnam in the 60's-70's (thanks Mr. Truman...).


Always, there is a politician who can be

thanked for a war. In Europe, Korea, Vietnam, Afghanistan, Serbia, Iraq…



As for ‘friendship’ and getting roped now everything is simpler. An attack at any

member of NATO means an attack at all others according to the treaty.