The meeting of the two forces took place on the evening of August 12. By this date, there was no longer any doubt that the Soviet armies had begun the encirclement of Warsaw - at least in the eyes of foreign observers who used the staffs of the Franco-British missions as a kind of "Second Bureau". The movement was led by the Kavkor,
which proved less brilliant during the critical days of August 6-17 than during its spectacular 720 km advance in July.
Radcliffe described the Kavkor at this time as "very flimsy and badly commanded as nothing would have prevented it from bypassing the left of the Polish army and cutting it off from Warsaw".
He indicated that the Kavkor marched only slightly faster than the Soviet formations.
Radio messages picked up by the Poles hinted at hostility, or in any case a lack of cooperation, between Ghai and the commander of the Soviet Fifth Army.
Despite everything, the Poles, much outnumbered along the Wkra, had reason to be worried. Weygand kept repeating that this line had to be held to the north before launching any counter-offensive.
He rejoiced with Radcliffe that Sikorski had been placed there and that the latter had received the 18th Division recovered to the south as reinforcements.
Warsaw was defended by seven divisions, strong artillery and fortified positions extending over 70 km; however, the attack by the Soviet XVI Army (five divisions) was quickly crowned with success. On the morning of August 13, the Russians launched an assault on the first Polish line of defence. They took Radzimir (24 km from Warsaw) and broke through the Polish division.
On August 14, there was hand-to-hand fighting, but the Polish command regained confidence when they realized that they were dealing with only one Soviet army. The next day the Poles threw into battle all their reserves, including the tanks which only stopped when they broke down, and they retook Radzimir. At Warsaw's Mokatow airfield, Polish mechanics frantically rounded up newly arrived English Bristol fighters to prevent any attempt at aerial reconnaissance by the Soviets.
The man who faced the rest of the Soviet army was Sikorski. Unaware of the larger forces in front of him, the commander of the Warsaw garrison had called him to help during the panic of the 13th. Sikorski attacked a day later, on the 14th. cavalry reached Ciechanow where they got their hands on the Soviet cipher maps and messages that were in the burnt-out radio set of Fourth Army HQ. On August 16 Sikorski continued his advance with tanks and eight armored vehicles. Behind him came two armored artillery trains as reinforcements, but he was forced to advance with his left flank uncovered. The Kavkor could have attacked it at this time but Ghai contented himself with bombarding the railway lines 65 km further on, on the other side of the Vistula around Wloclawek - the commander of the 3rd cavalry corps thus hardly showing himself cooperating with the Fourth Army.
After the panic of the 13th, the launch of the great encirclement movement by the Wieprz had been brought forward. On the 12th, Pilsudski had left the capital to personally take command of the strike force. He was horrified at the poor condition of his men's equipment. He described those in the 21st Division as "virtually naked", however, noting that morale was not too low. His concern was heightened by the bad news from Warsaw.
He then decided to bring forward the attack by one day, which he fixed for the 16th. On August 15th, the tension rose when the news reached that Budyanny was advancing at will to the south and approaching Lvov.
It was only on August 16 that 19 Polish aircraft replied with a regular bombardment that lasted 3 days; they flew 200 sorties to slow the Soviet advance. As ammunition ran out, the planes literally attempted to ram the Red Cavalry formations.
On August 16 the strike force advanced directly on Mosyr's group. To their astonishment, they met virtually no resistance. All day, Pilsudski, in a vehicle of the leading units, drove them forward, but they seemed to be pursuing a phantom enemy.
The advance was rapid:75 km in 36 hours, but Pilsudski, far from being reassured, took certain precautions:guard, for I felt that traps and mysteries threatened us on all sides. »
The Soviet armies may have been about to encircle it. It must be said to Pilsudski's credit that he kept his cool and ordered the leading troops to
pass the deserted highlands. On August 17, his advance guard met the 15th Division (from the Warsaw garrison) in Minsk Mazowiecki, which had just made a sortie.
So when he returned to Warsaw on the 18th, Pilsudski was beginning to believe in the success of his daring maneuver, which had taken him 112 km in three days.
In Warsaw, the mood was still to worry. It was hard to believe in success when there were still so few prisoners, few weapons captured, and before it had been so hard to stand up. Moreover, Sikorski was still outnumbered and in a difficult position.
For the Fifth Army had continued to advance on August 17 and 18, inviting the Soviets in a way to crush it by sheer weight of numbers. But before the Bolsheviks could muster their forces to defeat Sikorski, the effects of Pilsudski's spectacular advance were felt. The Soviet army leaders began to realize that they would have to retreat or be surrounded.