BBC
"Enshrined in Article 51 of the UN Charter, the right of self-defence is accepted as a fundamental principle of international law. While aspects of this principle are disputed, it is universally agreed that a state can defend itself against an armed attack.

There is some debate as to the intensity that an armed attack should reach before a state can lawfully resort to self-defence. Most international lawyers would agree that rockets launched against civilians that disrupt the social life of part of a country constitute an armed attack for the purposes of Article 51"
http://www.bbc.co.uk/news/world-middle-east-20415886