The Mongols had not touched the city-state of Novgorod, as it was just out of range to north and west. In this period, the region is known as the Novgorod Republic. Novgorod was ruled by a Prince who was appointed or elected by a strong city council, rather than inheriting the role automatically at birth. The Republic was a stable medieval state with trade contacts all through Europe (the archbishop asked church fees to be paid in bolts of wool cloth from Flanders!). Traders on the Volga and Dniepr rivers passed through Novgorod, whose rulers had also intermarried with Swedish kings in the past. (Imagine if the Novgorod Republic, not Moscow of the Tsars, had become the core of future Russia. History would be quite different.)
In the 12th century just past, Sweden and Novgorod had many frontier battles over ports and trading posts on the Baltic and around Finland. The German-based Hanseatic League wanted complete domination over the Baltic’s ports, and it saw Novgorod as a trade rival. Further, Sweden no longer looked on the Russian city as Christian, as its ties to Rome grew stronger. In fact, its sorties into pagan Finland began to be styled as Crusades.
Novgorod chose to become a tribute-paying vassal of the Mongols in Sarai; it had become plain that not paying tribute was a very poor choice. All of the cities in its Russian neighbor-network had been burnt and depopulated. The region was as weak as it had been in a long time, so…enter the Teutonic Knights, the northern Crusaders that I have trouble speaking well of. They occupied several Novgorodian cities along Lake Peipus, the large lake along the border of Estonia. Novgorod had to act, so the Republic called back its strongest leader from exile.
Aleksandr Nevsky was the grandson of a Rus prince with strong Byzantine ties, representing both Kyiv and Vladimir, the strongest princedom of the region. Their family brought some of the grandeur of Constantinople into the Russian forests, building wooden onion-dome churches. Aleksandr had served as the Prince of Novgorod already (and been banished) by the time he was 20. In 1241, the city decided they needed him to defend against the Teutonic knights.
Nevsky chose to fight on the frozen lake itself, in April. In April, the lake was still frozen so solid that it could support thousands of men fighting on its surface! That’s a cold climate! During the battle, the Teutonic knights (with Estonians they had pressed into service) fought for two hours but began to find the surface too slick. Nevsky chose to bring a reserve force onto the lake at that time. Some legends say that the ice broke up and the Teutonic knights fell into the water, but this may have been Sergei Eisenstein’s cinematographic invention in 1938. (Personally I have a hard time believing that ice in April, even in the frozen Baltic North, could stand up under Daenarys Targaryen’s dragon-fire attack, especially after the third dragon crashed into the lake.) In any case, Nevsky won and the Teutonic knight did not choose to try Novgorod’s territory again.
Nevsky’s attitude toward the Mongols was that they were the best choice of masters on offer. He became their regional tax-enforcer, not just unwillingly an ally but openly a friend and supporter. The Mongol Khan supported him in becoming Grand Prince of both Kyiv and Vladimir, and he actively helped defend their rule against invaders. He saw Rome and its allies as a greater threat to his land than the Mongols, who merely wanted tribute. So in one of the weirder twists of this period, Christian Novgorod was saved from Christian Germans so that it could serve the Mongols.