Artwork for Halo Wars 2 appears to have been leaked by twitter user @lifelower. You can see the tweet below:
— lifelower (@lifelower) June 3, 2016
If real the images appear to confirm that Halo War 2 is going to be set after the events of Halo 5. You can see an Ark, complete with a damaged core, and the brutes are back. As for the year 2555 there is no flood on the ark and USCN has its forces on the Ark. Halo 5 takes place in 2558 and there was no sign of the Spirit of Fire, the crew is reported still MIA as of 2558. So this means that either the game is set during the events of Halo 5 or at the very least post Halo 4.
Microsoft announced Halo Wars 2 will be playable at E3 2016 during its weekly Halo blog post,
— lifelower (@lifelower) June 3, 2016