With the dramatic new trailer above, Tannenberg has left Steam Early Access. At the time of writing, the 64-player WWI shooter has attracted 241 concurrent players, and I was able to join a half-full match on a US server quickly after completing the 5.4 GB download. The European servers are more populated at the moment, but it's also the middle of a Wednesday in the US, so that's understandable.
Tannenberg comes from Verdun creators Blackmill Games and M2H, and has been in Early Access since late 2017. Where Verdun takes place on WWI's Western Front, Tannenberg shifts to the east, with Russian, Romanian, Austro-Hungarian, German, and Bulgarian armies.
The primary mode, Maneuver, sees teams attack and defend sectors. It's more Red Orchestra than Battlefield, with a focus on authenticity as the armies push and pull along a front line—the tutorial here explains the basics.