"Tin soldiers" refers to the members of the National Guard that President Nixon sent to the Kent State campus in order to suppress student unrest.
The "drumming" referred to the so-called "war drums" predicting the uprising of student radicals and Vietnam War protesters. At that time, the government was viewed as the "enemy" by much of the youth of the US, and radical groups like the Weather Underground and Symbionese Liberation Army were attempting to wage war within the U.S.
Gotta get down to it ("get down to it" means to actually do something, take measures to solve a problem, get down to business)
Soldiers are cutting us down (Soldiers are shooting/killing us)
Should have been done long ago (Students should have protested and fought back long ago)
What if you knew her
And found her dead on the ground
How can you run when you know? (Probably doesn't refer to Mary Vecchio, the girl in the photo, but more likely one of the two female victims who died in the shootings.)