Last chance to go back but I will be discussing spoilers from Halo 5, mostly the ending
So…That happened. Cortana has pretty much taken the entire galaxy hostage with the guardians and the AI across it. Based on this we’re going to have to re-evaluate when Halo Wars takes place and what it’s ending might be like.
It’s highly unlikely it takes place between Halo 5 and Halo 6. With the galaxy being a complete mess it’s very unlikely the spirit of fire or covenant would have time to fight with each other. What about after Halo 6? Unlikely now since something certainly would have happened to the ship when Cortana sent out her message across the galaxy to AI’s with Serena being onboard.
So that means that most likely it takes place, well, whenever but before Halo 5s ending. And furthermore that they don’t get home, unless the very end of Halo Wars is when Cortana sends out her message.