Halo Wars: Definitive Edition will be coming to Steam later this week. The release of the game on Steam was confirmed by Microsoft earlier today.
The Windows version of the game was already available since late 2016 but only for players who purchased the deluxe edition of Halo Wars 2. The Definitive Edition is an "enhanced version" of the original title, updated to run natively on the PC.
The game will not support cross-play between the Steam and Windows Store versions, likely because the multiplayer servers for the two versions run independently of each other.
Halo Wars: Definitive Edition launches on April 20th 2017 and will retail for a price of $20.00 in North America and €20,00 in Europe.