On September 1, 1939, both Nazi Germany and the Soviet Union invaded Poland. Germany and the Soviet Union were allies at the beginning of the war.
Apart from countless battles and skirmishes in which the Polish cavalry units fought dismounted, there were 16 confirmed[4] cavalry charges during the 1939 war. Contrary to common belief, most of them were successful. The first and perhaps best known happened on September 1, 1939, during the Battle of Krojanty. During this action, elements of the Polish 18th Uhlan Regiment met a large group of German infantry resting in the woods near the village of Krojanty. Colonel Mastalerz decided to take the enemy by surprise and immediately ordered a cavalry charge, a tactic the Polish cavalry rarely used as their main weapon. The charge was successful and the German infantry unit was dispersed.The same day, German war correspondents were brought to the battlefield together with two journalists from Italy. They were shown the battlefield, the corpses of Polish cavalrymen and their horses, alongside German tanks that had arrived at the field of battle only after the engagement. One of the Italian correspondents sent home an article,[5] in which he described the bravery and heroism of Polish soldiers, who charged German tanks with their sabres and lances. Other possible source of the myth is a quote from Heinz Guderian's memoirs, in which he asserted that the Pomeranian Brigade had charged on German tanks with swords and lances.[6] Although such a charge did not happen and there were no tanks used during the combat, the myth was disseminated by German propaganda during the war with a staged Polish cavalry charge shown in their 1941 reel called "Geschwader Lützow".[1] After the end of World War II the same fraud was again being disseminated by Soviet propaganda as an example of the stupidity of Polish commanders and authorities, who allegedly did not prepare their country for war and instead wasted the blood of their soldiers
You may wish to fact-check that, in particular when exactly did the USSR invade Poland.
Odd thing, also, Germany invades Poland and England declares war on Germany. USSR invades Poland and England does not declare war on USSR.
And, of course, the Katyn Forest still sheds tears.
[If asking a Pole from that time and place: "Just who won WWII?", the reply would be the USSR.]