
Given the following passage

"At the time of the Marburg Colloquy, Suleiman the Magnificent was besieging Vienna with a vast Ottoman army. Luther had argued against resisting the Turks in his 1518 Explanation of the Ninety-five Theses, provoking accusations of defeatism. He saw the Turks as a scourge sent to punish Christians by God, as agents of the Biblical apocalypse that would destroy the antichrist, whom Luther believed to be the papacy, and the Roman Church. He consistently rejected the idea of a Holy War, "as though our people were an army of Christians against the Turks, who were enemies of Christ. This is absolutely contrary to Christ's doctrine and name". On the other hand, in keeping with his doctrine of the two kingdoms, Luther did support non-religious war against the Turks. In 1526, he argued in Whether Soldiers can be in a State of Grace that national defence is reason for a just war. By 1529, in On War against the Turk, he was actively urging Emperor Charles V and the German people to fight a secular war against the Turks. He made clear, however, that the spiritual war against an alien faith was separate, to be waged through prayer and repentance. Around the time of the Siege of Vienna, Luther wrote a prayer for national deliverance from the Turks, asking God to "give to our emperor perpetual victory over our enemies".",

answer the following question. Note that the answer is present within the text.

Question: For Luther, what would justify war? 

national defence
