These two lines provide the basis for the significance of this song. Tin soldiers (National Guard) and Nixon (government) are coming after the students. They have no one left to protect them (on our own). It signifies the students commitment to standing up against this strong resistance.

After the killing of four student protesters at Kent State by National Guardsmen,that university became a symbol of student resistance against the Vietnam War.

Source: "Kent State." Violence in America, edited by Ronald Gottesman and Richard Maxwell Brown, Charles Scribner's Sons, 1999. U.S. History in Context.

Thesis Statement: This song is one of the last protest songs written during a period of strong opposition, by young people, to the authority, the war, and the government.

Crosby, Stills, Nash and Young's remarkable single is arguably the perfect protest song: moving, memorable and perfectly timed........ Ohio turned out to signify the end of the fertile period of political songwriting that had begun with Bob Dylan, rather than a thrilling rebirth.

Source:

Students today can’t imagine why the students didn’t just leave. They don’t see protesting as part of the First Amendment (Rosenburg, David. Slate Magazine. 4 May 2013.