Demonstrating at a mass rally
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Joining a march
they were used to spark protest against America's involvement in the Vietnam War
an act of war
They weren't. The Vietnam war lasted 8.7 years according to Wikipedia. That is 2x the length of WWII, and just as long as the Iraq war. So, the answer is that protest songs were largely ineffective in changing the course of the war.
The Lend-Lease Act enabled other countries to continue their war against Germany .
Gulf of Tonkin Resolution