Wikipedia’s entry on Joseph Byerle who is believed to be the only American soldier who also served with the Russian military during WWII:
On June 6, D-Day, Beyrle’s C-47 came under enemy fire over the Normandy coast, and he was forced to jump from the exceedingly low altitude of 120 meters. After landing in St. CÃ´me-Du-Mont, Sergeant Beyrle lost contact with his fellow paratroopers, but succeeding in blowing up a power station. He performed other sabotage missions before being captured by German soldiers a few days later.
Over the next seven months, Beyrle was held in seven different German prisons, escaping twice only to be recaptured. Beyrle and his fellow prisoners had been hoping to find the Soviet army, which was a short distance away. After the second escape, Beyrle was turned over to the Gestapo by a German civilian. Beaten and tortured, he was released to the German military after officials stepped in and determined that the Gestapo had no jurisdiction over prisoners of war.
Beyrle was taken to the Stalag III-C POW camp in Alt Drewitz, from which he escaped in early January 1945. He headed east, hoping to meet up with the Soviet army. Encountering a Soviet tank brigade in the middle of Januaryâ€”reportedly holding his “hands up and [saying], ‘Amerikansky tovarishch, Amerikansky tovarishch”â€”Beyrle convinced the brigade’s commanders to allow him to fight alongside the unit on its way to Berlin, beginning his month-long stint in a Soviet tank battalion, where his demolitions expertise was appreciated.
Be sure to read Beyrle’s account of his experiences.