Every so many years someone comes along convinced that America won't follow through to punish them.
Yet we've never backed down. In our entire history, America has never once said "Oh, we won't go to war over that."
The lesson will be repeated throughout all of time as often as is necessary, however. America will go to war. America will punish you, and America will win anytime we really want to win.
Unfortunately, we did just that.
We bailed out of Viet Nam in the face of political pressure from the leftists, leaving our former allies to be slaughtered. When the NVA and VC violated the terms of the fig-leaf "peace" accord they had signed, we did nothing.
America has had some successful military campaigns since then, but we have yet to demonstrate that we can maintain our resolve over time while enduring significant casualties.