In 1939, Nazi Germany and the Soviet Union invaded and occupied Poland, marking the beginning of World War II. The Soviets invaded and annexed much of eastern Poland, which had been part of the country since the Peace of Riga two decades earlier. Much of the northern section of this area was added to the Byelorussian SSR, and now constitutes West Belarus. The Soviet-controlled Byelorussian People's Council officially took control of the territories, whose populations consisted of a mixture of Poles, Ukrainians, Belarusians and Jews, on 28 October 1939 in Białystok. Nazi Germany invaded the Soviet Union in 1941. The defense of Brest Fortress was the first major battle of Operation Barbarossa.