One could cast a strong vote for Grand Funk Railroad's, "I Can Feel Him In The Morning", considered the best anti-war song ever, the lyrics are hard and straight to the point.

GFR, YouTube

"Take me out to the battlefield,

Let me hear the shells flying by

Let me hear them scream and cry

Whoa, can't you hear their dreams

As they tumble to the blood-soaked ground?

Whoa, hear them scream for shelter

From the world they never found."

Another great entry for the protest song catalog would be Grand Funk Railroad's, "People, Let's Stop The War" which can be found on their album from 1971, 'E Pluribus Funk'.

Grand Funk Railroad's, "I Can Feel Him In The Morning"