[1] In 1224, Wenceslaus married Kunigunde of Hohenstaufen,[1] third daughter of Philip of Swabia, King of Germany, and his wife Irene Angelina.

The Emperor demanded Wenceslaus and other rulers of the Holy Roman Empire to lend him part of their own troops for his war effort.

Wenceslaus led a group of princes who expressed their reluctance to divert any troops from the defense of their own territories, citing fear of invasion from the Duchy of Austria.

Wenceslaus managed to negotiate the expansion of Bohemia north of the Danube, annexing territories offered by Duke Frederick in order of forming and maintaining their alliance.

In June 1239, Wenceslaus and Otto left the Reichstag at Eger, abandoning the service of excommunicated Emperor Frederick II.

He gathered reinforcements from Thuringia and Saxony along the way, before taking refuge in Bohemia's mountainous countries whose terrain would reduce the mobility of the Mongolian cavalry.

[9][10][11] After their failure against Wenceslaus' army, the Mongols led by Baidar and Kadan turned away from Bohemia and Poland and went southward to reunite with Batu and Subutai in Hungary, who had won a pyrrhic victory over the Hungarians at the Battle of Mohi.

[12] On 15 June 1246, Frederick II, Duke of Austria, was killed in the Battle of the Leitha River against Béla IV of Hungary.

However imperial governor Otto von Eberstein had to contend with an Austrian rebellion, preventing immediate benefits from the annexation of the Duchy.

[14] The Privilegium Minus, the document which had elevated Austria to a Duchy on 17 September 1156, allowed for the female line of the House of Babenberg to succeed to the throne.

Vladislaus was declared a jure uxoris Duke of Austria and managed to secure the support of part of the Austrian nobility.

Ottokar had been enticed by discontented nobles to lead the rebellion, during which he received the nickname "the younger King" (mladší král).

In order to secure dynastic rights to Austria, Wenceslaus had another female Babenberg proclaimed Duchess and betrothed to his son.