In the early sixteenth century the Ottoman Empire had become the dominant power in the Middle East. When Suleiman the Magnificent (r.1520-66) became Sultan in 1520 the empire included modern Turkey, Syria, Northern Iraq, Israel, Palestine, Egypt, the west coast of Arabia, and the Balkan territory of southeastern Europe. During his reign Suleiman added southern Iraq, Kuwait, the eastern coast of Arabia, most of North Africa, Hungary, and the Crimea. The Ottoman Turks fought aggressive wars on all fronts against Christians and other Muslims. When Suleiman’s army defeated the Hungarian army (and killed King Louis II of Hungary) at the battle of Mohacs in 1526 many Europeans rightly feared a massive Turkish invasion. In the fall of 1529 Suleiman’s army did lay siege to Vienna (the main city of the Hapsburg territory). Archduke Ferdinand I, brother and later successor of Holy Roman Emperor Charles V, led the defense of Vienna.
Martin Luther had published On the War Against the Turks in April 1529 as an explanation of how to properly fight the Ottoman Turks. In short, he rejected the idea of a crusade for two main reasons. First, crusades were pilgrimages by which the penitent performed acts of satisfaction to deal with the temporal guilt of contritely confessed sins. Since Luther rejected this understanding of repentance as early as 1517, he could not endorse crusades as a proper religious activity. Second, the idea of a crusade confused the earthly and spiritual kingdoms. Popes called the crusades as holy wars to fight against Muslims, pagans, or heretics. Dr. Luther repudiated the notion that priests or pastors should call or lead such a war. [W. Perry Copus, Jr., “Luther, the Crusades, and Just War,” Logia 18, 4 (2009): 7-11; and my post here: Luther on Crusades]
While Martin Luther did reject penitential warfare, he affirmed the idea of a just war. In On the War Against the Turk he explained how a defense against Suleiman’s army could take place justly. First, the Turks had initiated an aggressive war, which Luther compared to robbery and piracy. Second, Luther explains that Christians must resist the Turks through spiritual means:
Every pastor and preacher ought diligently to exhort his people to repentance and to prayer exhort. They ought to drive men to repentance by showing our great and numberless sins and our ingratitude, by which we have earned God’s wrath and disfavor, so that he justly gives us into the hands of the devil and the Turks. (LW 46:171)
Dr. Luther made it clear that sinful Germans had earned God’s punishment and that the Turks could be a divine means to carry it out, even if the Turks were demonic pawns. However, this did not mean that the civil authorities should not defend their land from an aggressive attack. In fact, as the earthly authority, the imperial office had the obligation to do so. Additionally, if called upon by the proper civil authority, Christian men could fight against the Turkish aggressors in good conscience. As Luther wrote:
If there is to be war against the Turk, it should be fought at the emperor’s command, under his banner, and in his name. Then everyone can be sure in his conscience that he is obeying the ordinance of God, since we know that the emperor is our true overlord and head and that whoever obeys him in such a case obeys God also, whereas he who disobeys him also disobeys God. If he dies in this obedience, he dies in a good state, and if he has previously repented and believes in Christ, he will be saved. (LW 46:185)
Finally, Dr. Luther explained that only problems arise when the spiritual and earthly authorities act against their divinely-ordained purpose. That is, the emperor must deal with secular matters like war and clergy should never exhort Christians to fight as acts of piety.