expanded westward under the command of Batu Khan to subdue the western steppes and drive into Europe. Their western conquests included Volga Bulgaria, almost all of Alania, Cumania, and Rus', along with a brief occupation of Hungary. They also invaded Poland, Croatia, Serbia, Bulgaria, the Latin Empire, and Austria. During the siege of Kolomna, the Khagan's half brother Khulgen was killed by an arrow. Amid the conquest, Ögedei's son Güyük and Chagatai's grandson Büri ridiculed Batu, and the Mongol camp suffered dissension. The Khagan harshly criticized Güyük: "You broke the spirit of every man in your army... Do you think that the Russians surrendered because of how mean you were to your own men?". He then sent Güyük back to continue the conquest of Europe. Güyük and another of Ögedei's sons, Kadan, attacked Transylvania and Poland, respectively. Although Ögedei Khan had granted permission to invade the remainder of Europe, all the way to the "Great Sea", the Atlantic Ocean, the Mongol advance stopped in East Europe early in 1242, the year after his death. Mongol accounts would later attribute the drive's failure to his untimely demise necessitating Batu's withdrawal to personally participate in the election of Ögedei's successor. Batu, however, never reached Mongolia for such an election and a successor wouldn't be named until 1246. A likely reason the advance stalled and never regained momentum is that European fortifications posed a strategic problem that Mongol commanders were unable to surmount with the resources they had available. Conflict with Song dynasty In a series of razzias from 1235 to 1245, the Mongols commanded by Ögedei's sons penetrated deep into the Song Dynasty and reached Chengdu, Xiangyang and Yangtze River. But they could not succeed in completing their conquest due to climate and the number of Song troops, and Ögedei's son Khochu died in the process. In 1240, Ögedei's other son Khuden dispatched a subsidiary expedition to Tibet. The situation between the two nations worsened when Song officers murdered Ögedei's envoys headed by Selmus. The Mongol expansion throughout the Asian continent under the leadership of Ögedei helped bring political stability and re-establish the Silk Road, the primary trading route between East and West. India Ögedei appointed Dayir Baghatur in Ghazni and Menggetu noyan in Qonduz. In winter 1241 the Mongol force invaded the Indus valley and besieged Lahore, which was controlled by the Delhi Sultanate. However, Dayir Baghatur died storming the town,