Just war has two aspects jus ad bellum and jus in bello. Jus ad bellum governs whether a particular military conflict is justified, while jus in bello regulates proper action in war. I should note that I reject the concept international law, as it violates subsidiarity, as international organization aren’t sovereign and therefore can’t make law, and as law is and should be made by a government of a people for that particular people, making one set of laws applicable to differing peoples is harmful. So I am talking about moral law here, not ‘legal’ law.
Before we even get to just war, we must define war. War is conflict between nations, for a war to be a war it must be waged by a people against another people, not by a person; if a person is waging war on their own, they are simply committing murder, not committing war.
For a war to be just a state of war must be entered by the people. To do this a legitimate authority over a people must declare the war on behalf of the people he represents. Someone who is not an authority for his people can not declare a war. This declaration need not necessarily be a formal declaration of war. A surprise or pre-emptive attack may be just declaration of war depending on the circumstance. (I am unsure on the question of whether an illegitimate authority can justly declare war on the people he has authority over; it will require more pondering).
For a declaration of war to be just, it must meet three conditions:
First, it must be defensive, either in defense of your own nation, in defence of another nation, or in defence of justice. Defence is not used in its strictest sense, and goes beyond simply warding off an invasion. For example, an invasion to rescue a national citizen kidnapped while visiting a foreign nation would be a valid defence of the nation, while an invasion to stop mass murder or to punish the guilty may be a valid defence of justice.
Second, the war must have some real chance of success. If a war would have no realistic chance of success, then the war is unnecessary, and would therefore be unjust. A small chance of success is still a real chance.
Finally, war must be proportional. The expected benefits of a war must be greater than the expected evils of war.
Once in war jus in bello should be followed:
First two principles are necessity and proportionality, unnecessary violence is to be avoided and violence enacted should not be disproportionate to the goals.
The third is the avoidance of deliberately targeting non-combatants. Violence should only be enacted upon legitimate military targets.
Finally, there is the proper treatment of POW’s. It should be noted here, that spies, saboteurs, and the like are not POW’s and can be dealt with harshly.
Those are the basics of just war.
So, now we get to Breivik. I think Breivik did have a just cause for war; the rapes, violence, and slow genocide of his people by foreigners and hostile elites are just causes for war, but he was not carrying out a just war.
The first reason was that he was not engaging in war. He acted alone, not as a part of a people; there was no war, simply murder. As well, he was not a legitimate authority, so his act of ‘war’ could not be a legitimate declaration of war to begin a war.
Secondly, his actions had no real chance of success. Given that propaganda outlets are almost entirely in the hands of his enemies, the most realistic outcome was that his actions would actively hinder his cause.
He also failed to meet jus in bello principles. The targets of his attacks were not legitimate military targets. Given the nature of the conflict in Norway, I think a legitimate case could be made that the ruling elite and politicians are legitimate military targets, but the spawn of the ruling elites were not. He should have targeted the politicians, media, and bureaucrats, not their children.
A just war in Norway, would require the Norwegian people, or at least a significant minority of them, to have (or appoint) a legitimate authority to declare war on behalf of their community in order to expel (not genocide) the foreign invaders and remove the internal traitors supporting them from positions of power.
If this community can not be found, no amount of lone wolf attacks will matter. The Norwegian people will, sadly, have chosen their own subjugation and extinction.