The Mongol invasion of Europe
The Mongols attacked Eastern Europe with five distinct armies. Two of them attacked through Poland in order to protect the flank from Bela's Polish cousins, winning several victories. Most notably, they defeated the army of Duke Henry II the Pious of Silesia at Legnica. A southern army attacked Transylvania, defeated the voivod and crushed the Transylvanian armies. The main army led by Khan Batu and Subutai attacked Hungary through the fortified Verecke Pass and annihilated the army led by Denis Tomaj, the count palatine on 12 March 1241, while the final army under Batu's brother Shiban marched in an arc north of the main force. Prior to the invasion, King Bela had personally supervised the construction of dense natural barriers along Hungary's eastern border intending to slow the Mongol advance and obstruct their movement. However, the Mongols possessed specialized units who cleared the paths with great rapidity, removing the obstacles in just 3 days. Combined with the extreme speed of the Mongol advance, called "lightning" by a European observer, the Hungarians lacked time to properly group their forces.
Warnings and Hungarian preparation
In 1223, the expanding Mongol Empire defeated an group of semi- allied Russian city states, at the time known as Rus, at the Kalka River using their famous tactic of the feigned retreat under Subutai and Jebe. This was part of their great cavalry raid to explore the lands beyond their knowledge under the direction of Genghis Khan.
The defeated princes of Rus that were captured by the Mongols were crushed alive under a victory platform following the battle. At this time, the Mongols were purely an expeditionary force in Europe, and did not lay siege unto major cities such as Kiev until decades later under the direction of Genghis Khan’s successor, Ogedei.
Hungary had tried to convert the Cumans to Christianity and expand its influence over them for several decades beforehand. The Hungarian King Béla IV even began to use the title "King of Cumania". When Cuman refugees (ca. 40,000 people) sought refuge in his kingdom, it seemed that at least a portion of the Cumans had accepted Hungarian rule. The Mongols saw Hungary as a rival, and the Cuman migration to Hungary as a casus belli. In their ultimatum they also blamed Hungary for "missing envoys".
The Mongolian threat appeared during a time of political turmoil in Hungary. Traditionally, the base of royal power consisted of vast estates owned as royal property. Under King Andrew II, donations of land to nobles by the crown reached a new peak: whole counties were donated. As Andrew II said, "The best measure of royal generosity is measureless". After Béla IV inherited his father's throne he began to reconfiscate Andrew’s donations and to execute or expel his advisers. He also denied the nobles' right of personal hearings and accepted only written petitions to his chancellery. He even had the chairs of the council chamber taken away in order to force everybody to stand in his presence. His actions caused great disaffection among the nobles. The newly arrived and grateful Cumans gave the king more power (and increased prestige with the Church for converting them) but also caused more friction. The nomadic Cumans did not easily integrate with the settled Hungarians and the nobles were shocked that the king supported the Cumans in quarrels between the two.
King Béla began to mobilise his army and ordered all of his troops, including the Cumans, to the city of Pest. Frederick II, Duke of Austria and Styria, also arrived there to help him. At this moment, the conflict between Cumans and Hungarians caused riots and the Cuman khan—who had been under the personal protection of the king—was murdered. Some sources mention the role of Duke Frederick in inciting this riot, but his true role is unknown. Another possibility is that Mongol spies helped spread rumors of the supposed Cuman-Mongol alliance to cause panic, similar to what the Mongols had done in the invasion of Khwarezm. The Cumans believed that they had been betrayed, and left the country to the south, pillaging all the way. The full mobilisation was unsuccessful; many contingents were unable to reach Pest; some were destroyed by Mongols before they arrived, some by renegade Cumans. Many nobles refused to take part in the campaign because they hated the king and desired his downfall. The loss of the Cumans was painful for Béla, because they were the one army in Europe who had experience fighting the Mongols.