The Spartans had no slaves--they had their Helots held in a Serf like arrangement. You could not buy and sell Helots who were tied to their land for life. The Helots had to give up 50% of their crops to the Spartan state. The Sparatans did indeed develop their military ways to keep the Helots from revolting so your "guards" refrence is accurate. Spartans did not practice chattel slavery or own chattel slaves.
From the Helot's point of view that's just so much BS. The Spartans kept the Helots as slaves. The Celt's kept the Germans as slaves, albeit in tribal groups, but in slavery nonetheless.
The Iriquois understood the concept quite well having been the slaves of the Algonquins earlier. Their histories, when reduced to writing, all begin, "when we were slaves". So, too with the Jews!
Slavery is such a universal situation with humanity that anyone who denies that it existed at one time or the other among any group of humans is simply not to be trusted.
BTW, one of the little stories in the ancient Briton stories of Arthur has him showing Christian piety and kindness by hunting down another man's slave and returning the slave to the master.
Like I said, universal!