President Obama and Prime Minister Benjamin Netanyahu of Israel share responsibility for the strains in their relationship. But there should be no doubt about Mr. Obama’s commitment to Israel’s security. When he warns that an Israeli attack on Iran could backfire, and that “there is still a window” for diplomacy, he is speaking for American and Israeli interests.

Iran’s nuclear appetites are undeniable, as is its malign intent toward Israel, toward America, toward its Arab neighbors and its own people. Israel’s threats of unilateral action have finally focused the world’s attention on the danger. Still, there must be no illusions about what it would take to seriously damage Iran’s nuclear complex, the high costs and the limited returns.

This would not be a “surgical” strike like the Israeli attack in 1981 that destroyed Iraq’s Osirak reactor, or the 2007 Israeli strike on an unfinished reactor in Syria. Iran has multiple facilities, and the crucial ones are buried or “hardened.” Pentagon analysts estimate that even a sustained Israeli air campaign would set back the program by only a few years, drive it further underground and possibly unleash a wider war.

It would also cast the Iranian government as the victim in the eyes of an otherwise alienated Iranian public. It would tear apart the international coalition and undermine an increasingly tough sanctions regime, making it even easier for Iran to rebuild its program.

Israelis have every right to be fearful and frustrated. For too long the world ignored Iran’s misdeeds and shrugged off Israel’s alarms. But while President George W. Bush blustered and made no progress, Mr. Obama — with a sharp nudge from Israel and Congress — has had increasing success rallying the international community to isolate and punish Tehran.

Mitt Romney’s claim that “if Barack Obama gets re-elected, Iran will have a nuclear weapon” is purely cynical; his own prescription for “crippling sanctions” and “military options” barely differs from Mr. Obama’s policy. The president’s offer to negotiate with Tehran has made it easier to persuade others to ratchet up the pressure.

We don’t know if there is any mix of sanctions and diplomacy that can persuade the mullahs to abandon their nuclear ambitions. American officials are right not to overpromise. Iran is feeling the bite from stiff restrictions on its banking system, and the pressure and pain should rise significantly in coming months as the European Union imposes an embargo on Iranian oil imports.

Tehran’s recent offer to return to the negotiations is almost certainly another feint, but must be tested.

What if sanctions and diplomacy are not enough?

Mr. Obama has long said that all options are on the table. In recent days his language has become more pointed — urged on, undoubtedly, by Israel’s threats to act alone.

Last week he told The Atlantic, “when the United States says it is unacceptable for Iran to have a nuclear weapon, we mean what we say.” In a speech on Sunday to the American Israel Public Affairs Committee, he declared that his policy is not to contain Iran, it is “to prevent Iran from obtaining a nuclear weapon.”

The United States military is far more capable of doing serious damage to Iran’s facilities than the Israeli military, but the cost would still be high, with many of the same dangers and uncertainties.

Mr. Obama is right that military action should only be the last resort, but Israel should not doubt this president’s mettle. Neither should Iran.