It is talking about the separation of Britain and America and it was written by Thomas Paine.
Paine thought that America was the one getting more affected in this situation, Britain was constantly at war and America would always supply them by sending people to the first lines for a certain death.
When he says "The blood of the slain, the weeping voice of nature cries, 'tis time to part." he is refering to the pain and suffering America has endured over the years and, so, it is time to move and find an individual, free and peaceful way.
Paine's writings had a big influence in American revolutionaries, his books meant a big inspiration even for radical philosophers and working-class people. He even influenced British thinkers like William Cobbett, George Holyoake, Charles Bradlaugh, Christopher Hitchens, y Bertrand Russell.