The sudden arrival of the Mongols compelled the Kievan Rus’ confederation, in an increasingly rare moment of solidarity (funny how the perception of an existential threat can bring people together), to muster a large army with contributions from each of its component principalities, including the three most important: Kiev, Galicia–Volhynia (the western part of modern Ukraine), and Vladimir-Suzdal (the eastern part of the confederation, which eventually became the core of Muscovy). This army may have been as large as 80,000 men by some modern estimates, though others put it at something closer to 30,000 and the surviving Russian sources aren’t very helpful in sorting it all out. The Mongols actually had a chance for reinforcement at this point, as there was another Mongolian army on campaign not too far away to the east. At some point, however, it became apparent that no reinforcements were arriving, and so Jebe and Subutai opted to send an embassy to the Kievan prince Mstislav (not to be confused with the Galician prince, who was also named Mstislav). Mstislav (the Kievan one) had the Mongolian ambassadors put to death, and if you know anything about Mongolian history you know how seriously they took that kind of thing.