The regional fallout from an Israeli attack might be the biggest negative factor. Israelis expect that thousands of missiles might be fired at their cities by Iran’s clients in Lebanon and the Gaza Strip, while U.S. forces might be attacked in Afghanistan, Iraq or in the Persian Gulf. But while the Pentagon worries about managing a fight on multiple fronts, Israeli leaders think they could handle their threat. Barak predicted last week that Israel would suffer fewer than 500 civilian casualties.

The most interesting calculations of all concern U.S-Israeli relations. The rupture of the U.S.-Israeli alliance arguably would be as large a blow to Israel’s security as Iran completing a bomb — and a unilateral attack might just risk that. The Pentagon might suspend what is now close cooperation; in Congress and in public opinion, Israel might be blamed for any U.S. casualties in Iranian counterattacks. I’ve always supposed that there will be no Israeli attack without a green light from Washington.

Israel, however, has a history of ignoring U.S. opinion at moments like this. It struck nuclear reactors in Iraq in 1981 and in Syria in 2007 with no American go-ahead. In both cases, there was no serious damage to relations — and, for that matter, no regional reaction.