And the Polish-Lithuanian invaders who did not hesitate for a moment to take as much as they could while Rus fought Mongols. The fact that Polish-Lithuanians were allies of Mongols in the Kulikov Battle dwarfs in villainy the notorious Molotov-Ribbenthrop Act.
Don't think the Mongols reallly needed much help in this.
Five years after the fall of Kiev, Papal envoy Giovanni di Plano Carpini wrote:
"They destroyed cities and castles and killed men and Kiev, which is the greatest Russian city they besieged; and when they had besieged it a long while they took it and killed the people of the city. So when we went through that country we found countless human skulls and bones from the dead scattered over the field. Indeed it had been a very great and populous city and now is reduced almost to nothing. In fact there are hardly two hundred houses there now and the people are held in the strictest servitude." Refugees from this death and devastation in Ukraine fled west area towards Poland.
As a result of the destruction of the cities and the tribute demanded by the Mongols, the lack of resources brought Ukraine to a standstill. With the Mongols most strongly situated in the Ukrainian southwest, the northeastern Russian cities gradually gained more influence - first Tver, and then, around the turn of the 14th century, Moscow. It wasn't until 1480 that Moscow became strong enough to finally throw off Mongolian rule - the Mongols contributed to the ascendency of Moscow over Kiev.
Sigh. This is the original Catastrophe from which the Ukraine never really recovered. Maybe it can finally have a chance....