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.