From The Council on Foreign Relations:
"The doctrine originated with the 1907 Hague Conventions, which govern the laws of war, and was later codified in Article 49 of the International Law Commission's 1980 Draft Articles on State Responsibility (PDF). The doctrine is also referred to indirectly in the 1977 Additional Protocols of the Geneva Conventions. Regardless of whether states are party to the treaties above, experts say the principle is part of what is known as customary international law. According to the doctrine, a state is legally allowed to unilaterally defend itself and right a wrong provided the response is proportional to the injury suffered. The response must also be immediate and necessary, refrain from targeting civilians, and require only enough force to reinstate the status quo ante. That said, experts say the proportionality principle is open to interpretation and depends on the context. 'It's always a subjective test,' says Michael Newton, associate clinical professor of law at Vanderbilt University Law School. 'But if someone punches you in the nose, you don't burn their house down.' "
From The New York Times:
GAZA — Waves of Israeli aircraft swooped over the Gaza Strip on Saturday, firing missiles at Hamas’s security headquarters and killing more than 200 people, bringing the highest death toll in Gaza in years in a crushing response to rocket fire by Hamas against Israeli towns.
After the initial airstrikes, which also wounded about 600 Palestinians, dozens of rockets struck southern Israel. Thousands of Israelis hurried into bomb shelters amid the hail of rockets, including some longer-range models that reached farther north than ever before. One Israeli man was killed in the town of Netivot and four were wounded, one seriously.
A military operation against Hamas, the militant group that controls Gaza, had been forecast and demanded by Israeli officials for weeks, ever since a rocky cease-fire between Israel and Hamas broke down completely in early November and rocket attacks began in large numbers against Israel. Still, there was a shocking quality to Saturday’s attacks, in broad daylight on about 100 sites, as police cadets were graduating, women were shopping at the outdoor market and children were emerging from school.
The center of Gaza City instantly became a scene of chaotic horror, with rubble everywhere, sirens wailing, and women shrieking as dozens of mutilated bodies were laid out on the pavement and in the lobby of Shifa Hospital so that family members could identify them. The vast majority of those killed were Hamas police officers and security men, including two senior commanders, but the dead included several construction workers and at least two children in school uniforms.