Although this was rejected by the Emperor, Melanchthon improved it as a private document until it was signed at a meeting of the Schmalkaldic League as the 1537 Apology of the Augsburg Confession, but the Catholic side did not respond to it until the 1545–63 Council of Trent.

In turn several Lutheran states led by Elector John Frederick I of Saxony and Landgrave Philip I of Hesse met at the town of Schmalkalden, where they established the Schmalkaldic League in 1531.

But, in 1544 Charles V returned to Germany from the Italian War after he had signed the Treaty of Crépy and began to forge alliances not only with Pope Paul III but also with Lutheran princes, foremost with Duke Maurice of Saxony, the Albertine cousin of Saxon Elector John Frederick I.

However, plans to invade Austrian Tyrol in order to bar the Emperor from bringing up Italian troops did not meet the approval of the Schmalkaldic princes.

Both Duke William IV of Bavaria and the Austrian Archduke Ferdinand I of Habsburg declared themselves neutral in the conflict, allowing Charles V to concentrate a mighty Imperial army without disturbances.

On 20 July 1546 Elector John Frederick I and Landgrave Philip I were placed under the Imperial ban, under the pretext that they had deposed the Catholic Duke Henry V of Brunswick-Wolfenbüttel in 1542.

In Swabia the Hessian troops took no further action, while the forsaken Imperial cities like the Lutheran princes Duke Ulrich of Württemberg and Count Palatine Frederick II chose to submit to the Emperor.

Due to disagreement in strategy, the League's defenses were finally routed on 24 April 1547 at the Battle of Mühlberg, where John Frederick I was taken prisoner.

In the case of Bremen, 12,000 imperial soldiers under the command of Duke Eric II of Brunswick-Calenberg unsuccessfully laid siege from January until May.

Charles V had to flee from the superior Lutheran forces and to cancel the Interim with the Peace of Passau, whereby John Frederick I of Saxony and Philip I of Hesse were released.