Mr. Obama’s prior statements—that containing a nuclear Iran is not an option; that a country committed to wiping Israel off the map, promoting terrorism and arming Hezbollah and Syria can’t be allowed to have nukes—have been strong. But Iran’s leadership still doesn’t seem to believe that an American military option really is on the table. …

Were Mr. Obama to affirm America’s dedication to blocking Iran’s nuclear ambitions through military force if necessary, he would maintain his flexibility to act while putting pressure on Iran’s mullahs. He would not be acknowledging, as some fear, that the combination of sanctions and diplomacy is failing. Rather, he would make this combination more effective by convincing Iran’s leaders that there is no good reason for them to continue bringing the economic pain of international sanctions onto their country. The message is that their sanctions-provoking projects are pointless because the U.S. will never allow Iran to become a nuclear power.

A policy of sanctions, diplomacy and an absolute dedication to the use of force if necessary has a far better chance of working than sanctions and diplomacy alone. Sanctions have certainly made life difficult in Iran, at least for the general population, but they haven’t slowed the regime’s nuclear march. Meanwhile, Israeli leaders have been forced to consider unilateral action in the absence of America’s clear commitment to stopping Iran before it’s too late.