Vietnam:
http://en.wikipedia.org/wiki/Gulf_of_Tonkin_Incident
I have to point out that prior to the full scale escalation of the Vietnam War, the US was in full support of the French colonization of Vietname. Seems a little hypocritical doesn't it? And even after the French left, they instated a puppet capitalist Roman-Catholic government, even though a majority of the population was buddhist, This too was supported by the US i believe (though i might be wrong on that).
Mexico:
http://en.wikipedia.org/wiki/Mexican_War
"Mere days before Polk intended to make his request to Congress, he received word that Mexican forces had crossed the Rio Grande and killed eleven American soldiers. On April 24, 1846, Mexican cavalry had attacked and captured one of the American detachments near the Rio Grande. After the border clash, battles between Mexican and American troops followed at Palo Alto and Resaca de la Palma.
Polk now made this the casus belli, and in a message to Congress on 11 May 1846 stated that Mexico had "invaded our territory and shed American blood upon the American soil."
He did not point out that the territory in question was disputed, and did not unequivocally belong to the United States. "
BTW, getting someone else to fight for you is a smart thing? maybe, but to encourage terrorism in nicaragua by illegally funding the contra-revolutionaries after it was banned? that just sounds dodgy.
In short, when it came to vietnam, N. korea, russia/afghanistan, the US was intervening only to protect capitalism. If you're gonna say that was a right move, then thats a whole different topic, I won't go into left/right arguments here, this is starting to stray a lil.