The secret protocol of the Molotov–Ribbentrop Pact, signed on August 23, 1939 defined the boundary between the German and Soviet "spheres of influence".

In 1 of September 1939 Germany decided to execute the secret protocol and attacked Poland from west. When Poland was mobilizing its army against nazis, Russia stabbed them behind their back and marched in from east in 17 of September 1939.
Commander of the Polish forces who defended Brest against the Germans, General Konstanty Plisowski, was arrested by the NKVD, sent to the Soviet prison camp in Starobielsk and later executed in the Katyn massacre.

In 23 of September 1939 the allies celebrated their common victory with parade in Brest-Litovsk.
Parade was leaded by Heinz Guderian (by German side) and Semjon Krivošein (by Russian side).

Guderian (center) and Krivošein