Polish infantry advance during the Battle of Warsaw.
August 17 1920, Warsaw–While the Soviets were attacking Warsaw’s defensive lines directly, the main Soviet thrust was north of the city, hoping to outflank the city and approach it from the west. There, they encountered Sikorski’s Fifth Army, which they vastly outnumbered. Nonetheless, Sikorski’s cavalry was able to capture the Soviet Fourth Army’s headquarters at Chiechanów (its commander getting away by car at the last second) on August 15, and on August 16 was able to hold its ground and prevent a retreat in which it likely would have been overwhelmed.
As Tukhachevsky’s attention was drawn increasingly toward Sikorski, on August 16 Piłsudski launched his planned counteroffensive from the Wieprz river, well to the south of Warsaw. He anticipated hitting the left Soviet flank, but with the bulk of the Soviet forces further east, Piłsudski was essentially attacking into nothing. They advanced 20-30 miles on the first day, and a similar amount on the second. Piłsudski began to second-guess himself: “Was I dreaming now that five divisions were merrily marching unopposed over the same area they were so recently abandoning to the enemy in the deadly terror of the retreat? This was a happy dream, but could it be real?”
On the 18th, however, Piłsudski‘s forces reached the flank of the Soviet Sixteenth Army, facing Warsaw’s defense lines. Faced with attacks from the front and on their flank, the Soviet units shattered and began to retreat, only to run into Polish units further east that were continuing their northward advance.
Tukhachevsky did not even hear of the Polish offensive until late on August 17, and believed it to be a diversion from the main fight with Sikorski. Contact with his units was limited at best, in part because the Poles, having received more than enough valuable intelligence from Soviet radio communications they had been able to decrypt, decided to start jamming Soviet communications instead. He hoped to regroup north of Warsaw for another attack on the city. Kamenev even called for an immediate attack on Warsaw, assuming that the capture of the city would end the war immediately and the sacrifice of the Sixteenth Army would be worth it. All of these ideas were ludicrous given the continued northward advance of Piłsudski‘s forces that threatened to cut off the bulk of the Soviet armies, and on August 20 Tukhachevsky ordered a general retreat.
Sources include: Adam Zamoyski, Warsaw 1920.
Polish advance, 1920.