Well yea, most slaves came from war. The winning king puts the target nation into debt for opposing him in the first place. You take their people as slaves, and they can't speak your language, so it's easy to dehumanize them or for them to escape.

The children of slaves, especially if they look similar to your people, usually do learn the language and it's much easier for them to escape and blend in. In many cultures, the children of slaves weren't even put into slave status.