When is war permitted?

Luther:

There are three types of war: from bottom to top, between two equal parties and from top to bottom. The first may never take place. You may never rebell, because the Scripture says that you have to obey the government. Therefore the third opinion is allowed. The government is allowed to quell rebellions. In wars of two equal parties there are wars for conquest and wars for defense. The first are from the devil, and God grants those who start this type of war no happiness. The others stem from human misery, and may God help them.