101, a little history lession for you. Things are never as simple as you think.

http://www.socialstudieshelp.com/Lesson_72_Notes.htm

As for WWII

http://www.socialstudieshelp.com/USRA_WWII_Begins.htm

Saving a little time here the civil war was fought to preserve the union.

Aix and it would seem yourself want to belive the reason is something else but it just isn't so.

We entered Vietnam because we wrongly felt the if Vietnam fell to communism, all of Southeast Asia would. This has been proven to be incorrect.

Reasons for the Korean war

http://www.fsmitha.com/h2/ch24kor.html

I will not even bother with explaining anything about the revolutionary war. Because if you really think that was the reason then there just isn't any use to.
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