Even slaves knew the cause was slavery

This is fascinating. The slaves and their owners knew what the Civil War was about and the former weren't taking any chances in case the North, less certain about the South's reasons for secession, got wobbly.

Many of the runaways had their own good explanation: they understood, even if many whites still did not, that this war might soon turn into a struggle for their freedom. In late August, a correspondent for the New-York Tribune interviewed two recently escaped Virginia slaves who were passing through Philadelphia. The white journalist was astonished to find the two black bondsmen quite well-informed on current war news and political developments. He added:

They say all the negroes now want is for our Government to arm them. Let such a fact be once generally known — the willingness of [Abraham Lincoln] to receive, train, and arm them — and he can have, almost immediately, more men than [Jefferson] Davis has or ever can muster. They are longing to be permitted to fight for freedom. They fully understand the nature of the contest going on.

Even far below the Mason-Dixon Line, on the isolated cotton plantations of the Mississippi Delta, the struggle over secession had fanned the embers of the slaves’ long-smoldering dreams of freedom. “The runaways are numerous and bold,” wrote a Vicksburg woman on June 19. “The house servants have been giving a lot of trouble lately — lazy and disobedient,” she recorded on another occasion, noting that they seemed to be anticipating a Union decree of emancipation when Lincoln reconvened Congress for its emergency session on July 4. (Independence Day, she noted with relief, came and went without any such inconvenient liberation.)