Rawls asserts that there are 8 principles of justice among free and democratic peoples. Two principles seem contradictory (see Law of Peoples, p. 37).
5. Peoples have the right of self-defense but no right to instigate war for
reasons other than self-defense.
8. Peoples have a duty to assist other people living under unfavorable
conditions that prevent their having a just or decent political and social
regime.
For #5 and #8, Rawls has left unsettled why peoples should defend other people living under unfavorable conditions if peoples only have a right to wage war in cases of self-defense. If a commonwealth defends people living under unfavorable conditions, then they have violated principle #5.
I assume Rawls will clarify the issue later, but I am wondering whether principle #8 should be tweaked in such a way to say that peoples have a duty to defend others only in cases where their freedom is threatened. For example, if some group invaded Canada, the United States has a duty to defend Canadiens against the aggressors because the aggressors threaten the safety and well-being of every American. The proximity, both geographically speaking and socio-politically speaking, of Canadiens gives us a reason to defend ourselves against the invaders. In this case, we may assume that the aggressors invaded a country with similar values. So, if they have attacked a country with similar values, then the invaders have no reason not to attack us. Going to war against the Canadien invaders would be an act of self-defense. Therefore, according to principle #5, Americans have a right to wage war since it is an act of self-defense.
The worry is how far we can extend an argument of self-defense to cover not only proximity issues but also more general topics, e.g., natural resource governance.