In President Obama's best-case scenario, an attack on Syria looks like this: a few dozen cruise missile launches, maybe some aircraft carrying out precision strikes. One worst case? A regional conflagration in which Iran comes to Syria's defense both there and elsewhere in the world. Which occurs is largely up to Iran, a country that is sending broadly divergent signals — probably on purpose.

The struggle over how to respond to the August 21 chemical weapons attack in Syria has been an overt proxy for our tense relationship with Iran for some time. We must respond because "Iran is hoping [we] look the other way," as Secretary of State John Kerry told the Senate Foreign Relations Committee. At The Nation, columnist Bob Dreyfuss makes the case directly: the push against Syria "is not about Syria at all. It's about Iran — and Israel. And it has been from the start." And then there's lobbying juggernaut AIPAC, the American Israel Public Affairs Committee, which promises to "storm the halls on Capitol Hill" next week on behalf of action, according to Politico. "Ambiguity … invites aggression," one AIPAC member told Poltico — referring to the on-going international effort to curtail Iran's development of nuclear weapons. Last but not least, Senator John McCain, staunch advocate of sweeping engagement in Syria, told CBS News, "the Russians are all in, the Iranians are all in, and it's an unfair fight."

On Wednesday, Iran's new president indicated what his country's "all in" response might be. The Washington Post quoted President Hassan Rouhani's address to a key Iranian leadership group:

"If something happens to the Syrian people, the Islamic Republic of Iran will do its religious and humanitarian duties to send them food and medicine," Rouhani told the Assembly of Experts. He described the situation in Syria as "dire" and condemned "military attacks on countries in this region, especially on Syria."

That statement followed a public statement last week by one of Rouhani's predecessors, Akbar Hashemi Rafsanjani, who pinned blame for the August attack on Syrian leader Bashar al Assad, according to the Financial Times — a claim beyond even what Russian president Valdimir Putin wants to acknowledge.

"God bless the people of Syria . . . they were subjected to chemical weapons by their own government and now they have to expect a foreign invasion," Mr Rafsanjani, who heads the powerful Expediency Council, said last week at an event in the northern province of Mazandaran.

Iran is home to a large population of survivors of chemical weapons attacks, as The Guardian has reported, primarily victims of attacks launched by Iraq during the Iran-Iraq War three decades ago. Rafsanjani assumed the presidency shortly after the war ended, but was acting commander-in-chief for much of it.

The existing regime wasn't happy with his recent remarks, as the Financial Times explains.

The remarks, quoted by the semi-official ILNA news agency, sparked an uproar in Iran, where officials have accused Mr Assad's opponents of being behind the attack. They were quickly scrubbed from the news website. Later, Marzieh Afkham, Iran's foreign ministry spokeswoman, denied the comments, saying they were "distorted".

The Iranian government's crackdown on an argument even slightly sympathetic to the West is just the start. The Wall Street Journal reports on an apparent message from a leader in the Iranian military to militia members, asking they "attack the U.S. Embassy and other American interests in Baghdad in the event of a strike on Syria."

The Iranian message, intercepted in recent days, came from Qasem Soleimani, the head of Revolutionary Guards' Qods Force, and went to Iranian-supported Shiite militia groups in Iraq, according to U.S. officials.

In it, Mr. Soleimani said Shiite groups must be prepared to respond with force after a U.S. strike on Syria.

The United States has evacuated some embassy staffers in Beirut and Turkey, though it's not clear if that move was in response to the leaked communication. Iran denied the report, suggesting people "remember that relying on U.S. intelligence reports from anonymous officials will repeat the tragedy of Iraq."

On Thurday, Al Jazeera reported that the supreme leader of Iran stated that his country would "support Syria to the end."

Washington and its allies "are using the chemical weapon [allegation] as a pretext," and "are saying that they want to intervene for humanitarian reasons," Khamenei said on Thursday.

"The United States is wrong about Syria, and it is certain they will suffer... just like in Iraq and Afghanistan," Khamenei told members of the Assembly of Experts, the body that supervises his work.

That presumption by Khamenei isn't a big surprise. A lengthy profile of the leader in Foreign Affairs magazine this month (available online with a subscription) suggested that Khamenei believes that the United States has, for a long time, sought to undermine his nation's stability and leadership using similar tactics.

Any response from Iran following American strikes is uncertain, which certainly is to that country's advantage. The idea that even the Obama administration's "limited" engagement might draw Iran into a de facto war bolsters arguments of those on Capitol Hill already inclined to oppose any action. In other words, Iran may be bluffing, protecting Syria the best way it can as Washington prepares to vote.

Photo: President Rouhani. (AP)