Adams got the right idea. The spartans were introduced before we knew about the covenant. It was orignaly used for like impossible missions that reg. humans couldn't survive.
Zeke- "They weren't fighting the Covenant for along time, they were fighting a Rebellion, thats why the Spartans were created, then the Covenant appeared and started to glass the colonies, then the Spartans went from fighting abunch of Rebels to fighting the Covenant. Cause the first time they actually fought the Covenant was when Spartan 034-Sam stayed on the Covenant ship because a needler shot damaged his suit, and at that time, they didn't have their Mjolnir suits, so they had no sheilding, so Sam stayed on the ship and detonated the bomb they set inside of it."