I find power in art through reflection more than immediate action. A lot of how we perceive the past and the lessons to be learned from it are reading a historical account. Seeing an accompanying art piece that makes the text come alive. In the Vietnam example, the Napalm Girl photograph speaks to the horrors of chemical warfare in a way that is immediately palpable. Born in the USA by Bruce Springsteen gives an honest account of the nihilism of war veterans in a way that's otherwise hard to explain to teenagers. Without this media, the war would not have the same visceral nature to a millenial like myself, who had not lived through the experience.
Just because artist didn't stop the Vietnam war, didn't mean say hadn't shaped history. Art can be very powerful. Obviously artists didn't create the correct art to stop the war. Not that art is useless.
But David Hasselhoff tore down the Berlin wall!
Who's to say that artists didn't stop the Vietnam War? If they didn't someone should stand up and take credit.