By April 11th, 1241, most of the pieces were in place. Hungary was isolated. No help would be coming from Poland, nor Bohemia. The Holy Roman Emperor, under edict by the Pope, could not rally his troops. The Pope, while willing to call a Crusade, was unwilling to raise the excommunication of the Emperor. So no help was coming from Bela IV's liege lord. The Duke of Austria, Bela's vassal, was in almost open rebellion, and refusing to move to the Hungarian king's aid. The Kipchaks, or Cumans, who had fled to Hungary from the steppe when the Mongols invaded Russia, were rampaging through the country after Bela's vassals murdered their Quan, Kotien. Bela's military aides, Teutonic Knights, the Bishop of Ugolin, and the Hungarian chivalry were spoiling for a fight. And with his flank columns returned, Subodei was ready to give it to them.
The Hungarian army was approximately 100,000 strong. The Mongols' top strength was perhaps 60% of that. When Bela's army debauched from Pest, the Mongols retreated to the northeast, for three days, to the Sajo river. They took up position in the wooded hills, across the river, but left the valley below unoccupied. They also left the bridge across the river undefended. And then they waited.
The Hungarians soon appeared and threw some forces across the bridge. that night escaped prisoners told them the Mongols were in the hills. Secure in their wagon lager, the Hungarians did nothing. It was a mistake.
The attack opened next morning, but it wasn't the usual cavalry charge. Initially the Mongols, under Batu, took the Hungarians under fire with catapults firing rocks, flaming pots, and perhaps even gunpowder. After several hours of this, the Mongols attacked the advance party, and drove them back over the bridge. At that point, Subodei, who had built a bridge elsewhere, and crossed the river, attacked the Hungarian rear and flank, driving the Hungarian army into an encirclement. Countercharges failed, the Templars and Bishop of Ugolin going down in a hail of arrows. And the catapults kept firing.
The, the Hungarians spied a weakness in the Mongol lines, an opening. First in small groups, then in larger ones, the Hungarians began to flee. It turned into a rout. All control was lost. Bela himself fled for his life. It was a trap. Harrying the Hungarians like game on a great hunt, Mongol archers rode alongside the fleeing enemy. By the time the wreckage reached Pest, 70,000 Hungarians were dead. Bela fled south, a Mongol tuman in pursuit. Hungary was now leaderless. And Europe did nothing.
Shortly before Christmas, starving people near Buda, saw what appeared to be a miracle. A herd of cattle had wandered out onto the frozen Danube. They went out and rounded them up, driving them to the west bank of the river. The Mongols, now assured that the ice would hold their horses, launched an immediate attack and stormed Buda at Christmas. Hungary was now a Mongol possession.
With the coming of Spring, 1242, the Mongols fanned out. A column invaded northern Italy, getting as far as Udine. Another column raided Wiener Neustadt and rode around Vienna. And then, they disappeared.
Word had been brought by the YAM, the Mongol Pony Express, that the previous December, Uggedai Qa Quan, had died at the capitol of Karakorum. There would now be another Kuriltai to elect his successor. All the princes and generals were required to attend; the princes to jockey for the throne, the generals because the law required it. Since, in any case, the princes would be taking their privste armies with them, Subodei lacked the resources to continue what he had initially envisaged as an 18 year campaign. It was over.
The Mongols decided to abandon Hungary, but razed it to prevent or impede any hostile military action from the Europeans. They decided to keep Russia. They never came back.
Bela did, and returned to rule. He became known as the "Cuman" because he adopted many of their customs, and secured positions for them in his army. And the great Mongol campaign in the west ended not with a bang, but a death in Mongolia.
