 Given the gravity of the decision to go to war, only very serious threats can give us just cause to fight. So if just cause is satisfied, then you have weighty positive reasons to fight. Lacking just cause does not in itself aggravate the badness of fighting, but does make it less likely that the people killed in pursuing one’s war aims will be liable to be killed (more on this below, and see McMahan 2005a), which makes killing very hard to justify. Even if having a just cause is not strictly speaking a necessary condition for warfare to be permissible, the absence of a just cause makes it very difficult for a war to satisfy proportionality.