At the beginning of July, 38,000 men, 135 tanks and 225 aircraft were concentrated on the Japanese side of the border, east of Khalkhin-Gol. The Soviet and Mongol forces numbered only 12,500 men, but Zhukov had 186 top-class tanks and 226 armored vehicles. He was going to need it. The Japanese plan provided for the dispatch of a large force whose mission would be to envelop the Soviet left flank, cross the river and seize the highest point of the sector (Bain-Tsagan). Thus, while the bulk of the forces would attack, tanks at the head, the enemy frontal device, the marching wing would take it from the rear to destroy the salient on the east bank.
The Japanese timetable called for the offensive phase of operations to end in mid-July and for the campaign to be over before the autumn rains. On July 2, Japan opened hostilities with an attack on the relatively weak defensive positions on the eastern shore. By the end of the day, Japanese tanks and infantry were reaching the river at several points inside the 3rd line of Soviet defences. But Zhukov was far too shrewd a leader to commit his reserves prematurely. Early in the morning of July 3, Russian Colonel Afonin, Chief of the Mongolian Army Advisors, was inspecting the 6th Mongolian Cavalry Division at Bain-Tsagan when he ran into Japanese troops who had just come from cross the river by surprise on a pontoon bridge. Lacking the education and equipment of their Soviet masters, the Mongols were swept away.
The next morning, at sunrise, Zhukov could not ignore the dangers of the situation. A Japanese thrust southward would cut off the hard-fought Soviet forces from their bases. He then gave up the armored units, which constituted his only reserve:the 11th tank brigade with the mission of attacking from the north, the 7th mechanized brigade from the south, and the 24th motorized infantry regiment from the north. -west, through the Mongol units in disarray. In total, these armored formations lined up more than 300 combat vehicles, while on both banks of the river the Japanese did not have half of them.
In his 1939 memoirs, Zhukov noted that it was now impossible for him to delay his counterattack, as the enemy had seen the advancing tanks and was hastening to go on the defensive and have the aviation the columns of vehicles advancing in the open. For miles around, there wasn't even a bush to block the view.
The rapidity of the Soviet maneuver alarmed and soon demoralized the Japanese. By 7 a.m., all the bombers at Zhukov's disposal had begun their bludgeoning, and for the first time the soldiers of the mikado felt the weight of heavily structured Soviet heavy artillery. From 9 a.m., the advanced elements of the Russian armored vehicles arrived on the battlefield; the general attack was launched at 10:45 a.m. For lack of time, the Japanese had not been able to dig deep dugouts and their combat training against tanks generally left something to be desired.
They were going to pay the price. The battle raged all day, and by evening it was no longer the Russians that were threatened by the encirclement. Soviet artillery and aircraft and, to make matters worse, bombs destroyed the pontoon bridge on the Khalkhin-Gol. There were several hundred drowned among the Japanese who tried to swim back across the river. Komatsubara was lucky enough to be able to fall back with his staff, but almost all of the soldiers who had attacked Bain-Tsagan lay dead or wounded on the slopes of the hill. When the heaviest fighting ended, on the night of July 4–5, the Japanese had lost half of their tanks in Manchuria. And although the day of July 3 cost the Soviets a good hundred tanks, the Red Army had been able to exploit the glaring deficiencies of the Japanese in terms of field artillery and anti-tank.
But the Kwangtung army had no intention of giving up its adventure in Mongolia. By the end of July, it had doubled its strength in the sector and recovered, to reinforce its positions on the Khalkhin-Gol, the anti-tank units of the divisions which had not been engaged. August 10 saw the creation of the Japanese Sixth Army, under the command of General Ogisu Rippo. 75,000 strong, it comprised 2 fully-strength infantry divisions, the 7th and 23rd D.I., a Manchukuo brigade, 3 cavalry regiments, 182 tanks, 300 armored vehicles and 3 artillery regiments supported by more of 450 aircraft. After the failure of an attack launched on July 23 and immediately halted by Soviet bombardment, a general and definitive offensive was planned for August 24 on a front 70 kilometers wide. In the Russian camp, victory remained uncertain.
Powerful reinforcements had to be channeled through the mediocre communications network that linked the heart of Russia to the front. But Stalin felt that the country's international prestige was at stake, and that his new negotiations with Hitler - not very lenient towards the weak - had reached the critical stage. Neither blood nor money mattered anymore. “For Stalin,” wrote an ex-Red Army officer, “our losses had no meaning whatsoever.”
Throughout the months of July and August, the Soviets assembled 3 infantry divisions, 2 cavalry, 7 autonomous brigades including 5 armoured, to which were added artillery and aviation units. It was no small feat. The previous year, the Japanese had completed building a railway line arriving a few kilometers from the border; but the nearest railhead, the supply base of the newly created Soviet First Army Group, was 650 kilometers away. However, the future offensive of Zhukov required the transport, on the rudimentary tracks of Mongolia, of 55,000 tons of supplies including 18,000 tons of artillery ammunition...
The trucks were out of breath and their overworked drivers were overwhelmed by the heavy heat of late summer and the devastating sandstorms of Central Asia. The shortage of trucks was such that sometimes artillery tractors had to be used for transporting supplies.
Zhukov had his plan. Since the Japanese had attempted a vast encirclement maneuver, they were going to be offered a course in tactics. He organized his new forces into three groupings:north, south and center, and deployed fast and permanently available armored units on the wings. It would be ready on August 20, four days before the enemy.
Until then, Zhukov carefully concealed his plans and troop movements; he maintained the element of surprise by implementing ingenious camouflage. False radio messages ordering huge quantities of earthmoving equipment led the Japanese to think that the Russians were going to bury themselves there to spend the fall. Sound effects gave them the impression that heavy piling work was being done. The movements of the large armored and motorized units took place only at night, noisily covered by aerial bombardment and artillery.