Happy Wars dev Toylogic announced on Twitter yesterday that it will be ending its service for Xbox 360 and Windows 10 marketplaces beginning June 14 2018, but will still be active on the Xbox One.
The decision to pull Happy Wars from the two platforms was made after six years of service on the Xbox 360. Maintaining an online server for any game can be expensive, and after a while devs have to consider if the cost is worth it.
As reported by Polygon, the shutdown will occur on December 17, 2018. Players have until March 15 to transfer Happy Tickets (the game's currency which can be purchased with real money) to the Xbox One, where the game is still active.
Happy War's in-game events will be continue on Xbox 360 and PC until June 13 2018, with the Xbox One version continuing to host new in-game events after that date.
Players can find more detail about the shutdown on the game's FAQ page.