It means if two armies are fighting and the horses of one run away, the other has a right to take them.

And it is just the same if a slave runs away from the Secesh to the Union lines.
He is called a contraband, just the same as if he were an ox or a horse.
They wouldn't send the horses back, and they won't send us back." "Is dat so ?" said Uncle Daniel, a dear old father, with a look of saintly patience on his face.

"Well, chillen, what do you mean to do ?" "Go, jis' as soon as we kin git to de army," said Tom Anderson.
"What else did the generals say?
And how did you come to hear them, Tom ?" asked Robert Johnson.
"Well, yer see, Marster's too ole and feeble to go to de war, but his heart's in it.

An' it makes him feel good all ober when dem big ginerals comes an' tells him all 'bout it.

Well, I war laying out on de porch fas' asleep an' snorin' drefful hard.