In-game
Steiner will only show up if Reinhard Heydrich is victorious in the German Civil War, ruling a small nation (Attack Group Steiner.)
Trivia
The name of the region of Germany that will break away and Steiner will eventually take control of after Reinhard Heydrich wins the German Civil War, Angriffsgruppe Steiner (Attack Group Steiner), is a reference to the bunker scene from the 2004 movie Downfall, which had been subjected to countless parodies online (like this one) - in particular, Hitler's line "Der Angriff Steiners war ein Befehl!" (Steiner's attack was an order!).