On this date in 1939, German forces launched the invasion of Poland, code named "Case White". Within two days, England and France would honor treaty obligations and declare war on Germany. The Second World War in Europe was well and truly underway.
And no, the Poles never "charged tanks with cavalry", nor was the invasion the fifteen minute bloodless one-sided walkover that your eighth grade history textbook made it out to be. The Poles did employ cavalry formations, but so did the Germans and the Russians; somehow, however, Polish cavalry has entered the collective cosciousness as a symbol of military futility. For what it's worth, the US Army didn't make its last "boots & saddles" cavalry charge until January of '42, in the Philippines.