 Complete the following phrase:  During his reign in Vladimir, Yuri waged several wars against Volga Bulgaria and founded the fortress of Nizhny Novgorod on the Volga River to secure the area from Bulgarian attacks. He installed his younger brother Yaroslav in Novgorod. When the Mongols first approached Russia in 1223, he sent a small unit against them, but it arrived too late to take part in the disastrous 