Halo Spartans Don't Die, follows the story of some Spartan-II's and Spartan-III's that went MIA during their missions.
UNSC sent out search ships months after the Human-Covenant War in search of these MIA Spartans. Many were found.

The reason the UNSC needs Spartans, there are still renegade Covenant who still belive that the human race should be exterminated. The fraction is led by Brute Chieftians. They still call themselves The Covenant.

Another threat has also arised, Rouge Androids, these Androids were meant to replace the Spartan Program if no Spartans were found since Master Gunnery Sargent, John-117 went MIA at the end of the war.