The Battle of the Bzura was the single largest battle in the 1939 September campaign fought between September 9 and 19. It took place to the west of Warsaw, near the Bzura River. In it, a Polish breakout attack gained initial success but eventually faltered after a concentrated German counterattack.
It is also noted as one of the last major military actions ever to have been conducted on horseback.
Polish forces consisted of Army Poznań and Army Pomorze (8 infantry divisions and 2 cavalry brigades) totaling 225,000 soldiers. German forces included the 8th Army, 10th Army, elements of the 4th Army and the primary divisions of the German Luftwaffe (12 infantry divisions and 5 armored and motorized divisions) totaling 425,000 soldiers.
Only a few Polish units managed to break out of the encirclement. These groups entered Warsaw and Modlin around 19th and 20th of September.
Polish casualties and losses: 18,000–20,000 dead, 32,000 wounded and 170,000 captured.
German casualties and losses: 8,000 dead, 4,000 captured, 50 tanks, 100 cars, 20 artillery pieces.
[More about this battle at Wikipedia]
After Polish defeat in the Battle of the Bzura the Germans gained an undisputed advantage. Polish forces then began a withdrawal southeast, following a plan that called for a long defence in the Romanian bridgehead area, where the Polish forces were to await an expected Allied counterattack and relief.
