 Complete the following phrase:  In the later part of his reign, Ivan divided his realm in two. In the zone known as the oprichnina, Ivans followers carried out a series of bloody purges of the feudal aristocracy whom he suspected of treachery after the betrayal of prince Kurbsky, culminating in the Massacre of Novgorod in 1570. This combined with the military losses, epidemics, and poor harvests so weakened Russia that the Crimean Tatars were able to sack central Russian regions and burn down Moscow in 1571. Despite this, the next year the Russians defeated the Crimean Tatar army at the 