slaves... for an army.. wonder what would make them fight.

I think we also need to make the distinction between slaves and a conquered people. The vast majority of roman slaves went into labor, however romans routinely recruited units from the people that had just conquered. They were not "slaves" as how we think of them today, since they still retained many freedoms. Slaves as we think of them, don't make good soldiers. Though you can find a few examples, they are exeption and not the rule.

There have been plenty of slave-armies in history, but it depends on the definition of slave as such. Take the Mamluks. Techincally they were slaves to the Sultan and did his bidding, were sold and bought, but they also had a very high social rank, and ended up as the practical rulers of large parts of the Islamic world in the middle ages. Yet they were still considered slaves, and they themselves were proud of their slave history.

Read more on Wikipedia or such.

And as far as I know, some cultures did use conquered peoples in battle, but usually against the enemies of that particular people. Though I don't know if one would call these conquered people slaves as such or something else.

Practically, if an army uses slaves, they can't comprimise a very high amount of the army, and will mainly serve as arrow fodder and front line troops and such, as they are expendable.