Couldn't be much clearer that it was aimed at the Vietnam War, with that penultimate verse:

Well, maybe you'd rather not draw any moral;
I'll leave that to yourself
Maybe you're still walking and you're still talking
And you'd like to keep your health.
But every time I read the papers
That old feeling comes on:
Waist deep in the Big Muddy and the
Big Fool says to push on.

But of course sung today it means a different war.