Promote it in the abstract, or promote a very specific war.
The first seems inside the line to me (stuff like 'support our troops' or 'Join the army, take the fight to our enemy'). The second is past the line (stuff like 'we need boots on the ground in Syria, write your senator!').
I guess legally there is a difference between 'promoting violence between citizens' and 'promoting violence in war'. Since the first is usually illegal, and the second is not.
In general, government has a monopoly on legal violence. As such ads that promote war are promoting legal violence. Rules against promoting violence are (implicitly) targeting illegal violence.
At the same time, if FB reduced political adds (including any war/military related ones), I would be supportive.
That obviously seems to be promoting violence to me, but unfortunately, that doesn't seem to be a popular opinion.