In the course of the Lutheran Reformation numerous Imperial States had adopted the new confession, against the opposition of the ruling Catholic House of Habsburg, who recognised these conversions as a quest for increasing autonomy to the detriment of the central Imperial authority.

At the 1521 Diet of Worms Emperor Charles V had Martin Luther banned and the proliferation of his writings prohibited.
The edicts of the Diet condemned Luther and officially banned citizens of the Holy Roman Empire from defending or propagating his ideas, subjecting advocates of Lutheranism to forfeiture of all property, half of the seized property to be forfeit to the imperial government and the remaining half forfeit to the party who brought the accusation.
While it was understood that Luther was to be eventually arrested and punished, enforcement of this was suspended on account of the strength of his current popular appeal.
After the Diets of Nuremberg failed to accomplish the goal of arresting Luther, the Diet of Speyer (1526) reversed course and temporarily suspended the Edict of Worms.
This diet was condemned at the Diet of Speyer (1529), provoking the Protestation at Speyer and giving rise to the term "Protestant."
This led to the presentation of the Lutheran Augsburg Confession and Catholic Confutatio Augustana at the 1530 Diet of Augsburg.
In response to the Confutatio, Philipp Melanchthon prepared the Prima delineatio.
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.
At first, the Nuremberg Religious Peace of 1532 granted religious liberty to members of the Schmalkaldic League.
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. In view of the Emperor's preparations for battle, the Schmalkaldic leaders on 4 July 1546 gathered at Ichtershausen.
Here they negotiated how the league should deal with the approaching conflict with the emperor.
Both John Frederick and Philip of Hesse agreed quickly that the Emperor had larger financial resources and thus could set up a larger army.
However, they also noticed that they were positioned to mobilize their troops faster than the emperor because Charles V had not yet concentrated a significant amount of mercenaries.
As a result they decided to wage a preventive war.
Since Martin Luther had died in February, this eliminated a major obstacle to their decision.
Luther had repeatedly argued against the legality and morality of a war between the Empire and the Schmalkaldic League.
To Luther, only a beerwolf type ruler could be legitimately resisted by his own subjects.

The Emperor gathered an army of around 52,000 men (20,000 Germans, 12,000 Italians, 10,000 Spaniards, and 10,000 men from the Netherlands) for his campaign, that was to start on the Danube.
In the course of the Lutheran Reformation numerous Imperial States had adopted the new confession, against the opposition of the ruling Catholic House of Habsburg, who recognised these conversions as a quest for increasing autonomy to the detriment of the central Imperial authority.
At the 1521 Diet of Worms Emperor Charles V had Martin Luther banned and the proliferation of his writings prohibited. The edicts of the Diet condemned Luther and officially banned citizens of the Holy Roman Empire from defending or propagating his ideas, subjecting advocates of Lutheranism to forfeiture of all property, half of the seized property to be forfeit to the imperial government and the remaining half forfeit to the party who brought the accusation. While it was understood that Luther was to be eventually arrested and punished, enforcement of this was suspended on account of the strength of his current popular appeal. After the Diets of Nuremberg failed to accomplish the goal of arresting Luther, the Diet of Speyer (1526) reversed course and temporarily suspended the Edict of Worms. This diet was condemned at the Diet of Speyer (1529), provoking the Protestation at Speyer and giving rise to the term "Protestant." This led to the presentation of the Lutheran Augsburg Confession and Catholic Confutatio Augustana at the 1530 Diet of Augsburg. In response to the Confutatio, Philipp Melanchthon prepared the Prima delineatio. 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. At first, the Nuremberg Religious Peace of 1532 granted religious liberty to members of the Schmalkaldic League. 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. In view of the Emperor's preparations for battle, the Schmalkaldic leaders on 4 July 1546 gathered at Ichtershausen. Here they negotiated how the league should deal with the approaching conflict with the emperor. Both John Frederick and Philip of Hesse agreed quickly that the Emperor had larger financial resources and thus could set up a larger army. However, they also noticed that they were positioned to mobilize their troops faster than the emperor because Charles V had not yet concentrated a significant amount of mercenaries. As a result they decided to wage a preventive war. Since Martin Luther had died in February, this eliminated a major obstacle to their decision. Luther had repeatedly argued against the legality and morality of a war between the Empire and the Schmalkaldic League. To Luther, only a beerwolf type ruler could be legitimately resisted by his own subjects.
The Emperor gathered an army of around 52,000 men (20,000 Germans, 12,000 Italians, 10,000 Spaniards, and 10,000 men from the Netherlands) for his campaign, that was to start on the Danube.
Schmalkaldic War