Anti-Vietnam war and the ideology of the new left was a transnational movement across different countries in the 1960s. The Vietnam war was a focal point of student protests outside the US as an example of western imperialism. Students in cities of NATO countries like the UK, France, and West Germany all demonstrated against the Vietnam War, sometimes violently. So it's no surprise that popular music would be inspired by the war.

For further reading, please see "1968: The Year that Rocked the World" by mark kurlansky