On September 1, 1939 German troops crossed into Poland and the Second World War began. Britain and France, having guaranteed Poland's borders, would declare war on Germany in two days' time. The Soviet Union was, at that stage, Germany's ally and would, itself, invade Poland on September 17. Nobody guaranteed the country's eastern borders.
This is a well known picture: German soldiers move Polish border fences to open the road to tanks, armoured vehicles and for the troops to march in.
Below is a more optimistic picture: Poland for years under German and Soviet occupation became independent again in 1989. Today they were commemorating the German invasion and the tragedies that followed for many years.
The young people here in America do not know the suffering endured by the Polish people. Unfortunately, what they are being taught in the public schools about WWII is how the American government placed the Japanese-Americans in "concentration camps." I teach a course in World War history at a small very conservative college, and I see the students' 'indoctrination' at the hands of the school system first hand.
Unfortunately but understandably American view of the Second World War starts with Pearl Harbour. At least most Americans know that there was a good deal going on before that and some had come over to fight in the Eagle Squadron. In Russia they have absolutely no awareness of what happened before June 22, 1941.