The United Nations charter preserved the customary right of a state to retaliate against an “armed attack” from another state. The right has evolved to cover nonstate actors operating beyond the borders of the state claiming self-defense, and arguably would apply to Hamas. However, an armed attack involves serious violations of the peace. Minor border skirmishes are common, and if all were considered armed attacks, states could easily exploit them — as surrounding facts are often murky and unverifiable — to launch wars of aggression. That is exactly what Israel seems to be currently attempting.