Batu). This mission was more than just a typical invasion: Ögedei sent many of the next generation of Mongol princes, including the heirs of each of the four families, most likely to be trained by Subutai before he died. In order to crush the Volga Bulgars, who had previously defeated Batu in 1232, Subutai launched another giant encirclement campaign. The Mongols marched up the Volga river in a wide arc on the west side. However, this force was just to occupy their attention, for he sent a secondary army to surprise the Bulgars from the east by crossing the Ural mountains. After squashing the Bulgar armies, he defeated the guerrilla leader Bachman on the north side of the Caspian Sea. Bachman had ensconced himself on an island, and taunted the Mongols. However, they constructed a mini-navy of 200 boats, trapped Bachman within a limited area, and gradually closed the 'net'. Unlike in 1222–23 when the majority of the Rus' states allied against Subutai and Jebe, this time the Mongols apparently struck with such speed that the Rus' were either too paralyzed, or possibly too bitter or distracted to ally. In late 1237, Subutai attacked Ryazan and Vladimir-Suzdal, operating with three columns (attacking as the Mongols usually did during the winter, when the rivers froze over). The Rus forces were defeated in 3 separate engagements and their cities were taken in quick succession. The Mongols spent the summer of 1238 resting along the Don River. Columns were sent out to subjugate the various tribes living in the plains around the Black Sea. In 1239, the Rus' state of Chernigov was defeated and their cities were taken. The Mongols were spared the need to conquer Novgorod when the principality surrendered, agreed to pay tribute in the future, and gave the Mongols a large bribe. The most notable encounter of the campaign was the battle of the Sit River in 1238. The Grand Duke Yuri of Vladimir, the most prominent of the Rus' leaders, had left Vladimir to raise an army and defeat the Mongols before they reached his city. However, the Mongols evaded his army and captured Vladimir before he even knew what had happened. When Yuri sent out a strong reconnaissance force to penetrate the Mongol scouting screen, his lieutenant told him in horror that his army was already entirely surrounded. Not surprisingly, Yuri and his army were easily defeated. The