In the German town war of 1388, he assisted Frederick V of Hohenzollern, burgrave of Nuremberg, and in 1391 did the same for the Teutonic Order against Wladislaus II of Poland.

The danger to Germany from the Hussites induced Frederick to ally himself with Emperor Sigismund; and he took a leading part in the war against them, during the earlier years of which he met with considerable success.

In the prosecution of this enterprise Frederick spent large sums of money, for which he received various places in Bohemia and elsewhere in pledge from Sigismund, who further rewarded him on 6 January 1423 with the vacant electoral Duchy of Saxony-Wittenberg; and Fredericks formal investiture followed at Ofen on the 1 August 1425.

Thus spurred to renewed efforts against the Hussites, the elector was endeavouring to rouse the German princes to aid him in prosecuting this war when the Saxon army was almost annihilated at Aussig on the 16 August 1426.

After the death of his brother William, Frederick became the ruler over the entire possession of the House of Wettin except Thuringia.