Britain was a steadfast ally of America in the war to liberate Iraq. Prime Minister Blair articulated a clear understanding of the threat posed by Saddam Hussein and the opportunity created by his departure. British soldiers died in combat along with Americans in helping to free Iraq. Which makes all the more stunning the remarks the British foreign secretary, Jack Straw, made on Monday about Iran.

Mr. Straw, who was in Iran, told the BBC, “No one should ever compare Iran with Iraq in terms of their political systems or their danger.” President Bush had made exactly that comparison in his January 2002 State of the Union speech, which grouped Iraq and Iran together as part of the “axis of evil.”

Then the BBC interviewer asked Mr. Straw, “You talk in very reasonable terms about the diplomatic options. In this scenario you say no one should ever compare Iran with Iraq. Do I take it from that that there are no circumstances in which we would agree to an attack on Iran?” Mr. Straw replied, according to a transcript of the interview, “Yes and I can conceive of no such circumstances.”

It’s hard to think of a more foolish statement uttered by a more senior official in the recent history of statecraft. It amounts to a green light to Iranian terrorism and an instruction to the mullahs that any Western pressure regarding its nuclear weapons program can be safely ignored.

We’re not advocates of an immediate American military attack on Iran, though we wouldn’t rule out the chance that if the popular Iranian protests against the clerical regime, which are scheduled for July 9, are large enough, the effort to topple the mullahs could benefit from some American air raids aimed at key command and control targets. In other words, military force might help the Iranian people defeat their terror-sponsoring dictatorship.

Even the Europeans who are not interested in freedom in Iran or in counterterrorism, however, claim to be interested in preventing the proliferation of weapons of mass destruction. Iran has been actively pursuing nuclear weapons. Discouraging that pursuit — or at least encouraging international inspections — was the ostensible purpose of Mr. Straw’s trip to Tehran.

But if disarmament is the goal, a credible threat of force is necessary — or at the very least useful. It’s not just neo-conservatives and Donald Rumsfeld who believe this. Here is former President Clinton, writing in the Guardian in March 2003: “Only the threat of force from the US and the UK got inspectors back into Iraq in the first place. Without a credible threat of force, Saddam will not disarm.” Here is Mr. Straw himself, in November of 2002, in an official statement on the situation in Iraq: “The inspectors are returning because we have backed our diplomacy with the credible threat of force.”

If Mr. Straw persists in undercutting allied credibility with Iran, President Bush may want to revisit his decision to deal with Iran’s nuclear ambitions in partnership with Europe.