Somewhat unexpectedly Microsoft has confirmed that Halo: Spartan Assault will launch on Steam’s digital distribution service on April 5th. _ Halo: Spartan Assault was an exclusive to Windows Phone originally but later debuted on the Windows 8 Store. _It has also come out on the Xbox One and 360. Steam_and confirms that it will include 30 missions that not only deliver intense gameplay, but also explore the Spartan Ops program seen in Halo 4. _Special features include Steam achievements, leaderboards, and Steam trading cards, and Steam Cloud save games. _The cooperative mode has been stripped from this version of the game. Source: Steam

The rest of the story...
Microsoft News