Ottokar II campaigns in 1278 against Austria, supported by Duke Henry I of Lower Bavaria, who had switched sides.
Ottokar first lays siege to the towns of Drosendorf and Laa an der Thaya near the Austrian border, while Rudolph decides to leave Vienna and to face the Bohemian army in an open pitched battle in the Morava basin north of the capital, where the Cuman cavalry of King Ladislaus can easily join his forces.
Compelled to make terms with German king Rudolf I in 1275 but embarrassed by his extensive losses and his forced recognition of Rudolf’s suzerainty over Bohemia and Moravia, Ottokar rebels in 1278 and assembles an army in Prague.
Spurning Rudolf’s efforts to negotiate, Ottokar advances on Vienna.
Rudolf, his army reinforced with Alsatian and Swabian troops and aided by King Ladislas IV of Hungary, engages his opponent in the Battle of the Marchfield, near Durnkrut, on August 26, 1278, in a match of over eighty thousand men and the largest battle of knights in the Middle Ages.
After three hours of continued fighting on a hot summer day, Ottokar's knights in their heavy armor are exhausted, many of them suffering from circulatory failure and unable to move.
At noon Rudolph orders a fresh heavy cavalry regiment he had concealed behind nearby hills and woods to attack the right flank of Ottokar's troops.
Such ambushes are commonly regarded as dishonorable in warfare and Rudolph's commander Ulrich von Kapellen apologizes to his own men in advance.
Nevertheless, the attack prevails in splitting and stampeding the Bohemian troops.
Ottokar realizes the surprise attack and tries to lead a remaining reserve contingent in the rear of von Kapellen's troops, a maneuver that is misinterpreted as a rout by the Bohemian forces.
The following collapse results in a complete victory of Rudolph and his allies.
Ottokar's camp is plundered, and he himself is found slain on the battlefield.
