Philip of Hesse is, with John Frederick I, Elector of Saxony, a cofounder of the Schmalkaldic League, a defensive alliance of Lutheran princes within the Holy Roman Empire that has enabled the Reformation to take hold throughout Germany.

It had originated in 1531 as a defensive religious alliance, with the members pledging to defend each other should Charles V, the Holy Roman Emperor, attack their territories.

The League had quickly become more of a territorial political movement, as breaking from the Catholic Church offers significant economic advantages.

The league had in December 1535 admitted anyone who would subscribe to the Augsburg Confession, thus Anhalt, Württemberg, Pomerania, as well as the free imperial cities of Augsburg, Hanover, Frankfurt am Main, and Kempten have joined the alliance.

Francis I of France had joined the League against the Habsburgs in 1535, but later retracted due to religious conflicts from within.

It had allied in 1538 with newly reformed Denmark.

The League had in 1539 acquired Brandenburg, which is under the leadership of Joachim II Hector.

Philip, in effecting a bigamous marriage in 1540 to Margaret of Saale, has lost the support of many reformers, forcing him to make peace in 1541 with Emperor Charles.