What rule of war is that?
You can try the Lieber Code adopted in 1863. Article IX CONDEMNED assassination attempts:
The law of war does not allow proclaiming either an individual belonging to the hostile army, or a citizen, or a subject of the hostile government an outlaw, who may be slain without trial by any captor, any more than the modern law of peace allows such international outlawry; on the contrary, it abhors such outrage.