It's still too early to say whether Ayatollah Ali Khamenei and Mr. Ahmadinejad will succeed in their hard-line coup; de facto opposition leader Mir Hossein Mousavi remains publicly defiant. Yet it is becoming quite clear -- for all who care to see it -- what the Khamenei-Ahmadinejad regime will offer if it survives: harsh repression at home and unrelenting hostility toward the West. If the regime chooses to "engage" at all with the United States, it will be to bolster its shaky legitimacy, not to surrender its nuclear program or its support for terrorism. The only plausible path toward ending the threat it poses is that demanded by the demonstrators: regime change.

Some have theorized that Mr. Ahmadinejad's repression of the massive popular uprising could at least make it easier for the United States to build a coalition able to impose tough sanctions. But this week brought a depressingly familiar indication of how that diplomacy will unfold. Russia, which along with China has recognized Mr. Ahmadinejad as the election winner, blocked a Group of Eight meeting from even condemning the government's violence. "Isolating Iran is the wrong approach," said Russian Foreign Minister Sergei Lavrov, repeating his standard line. With U.S. support, the G-8 ended up renewing its invitation to Iran to open negotiations on its nuclear program -- even though the blood on Tehran's streets is not yet dry.

That stance would seem to contradict the position Mr. Obama took on Tuesday, when he denounced the regime's violence, said the protest movement was "on the side of history" and suggested that his policy of engagement would be put on hold. After meeting with German Chancellor Angela Merkel yesterday, Mr. Obama refined that stance, saying that while "multilateral discussions" with Iran could proceed on the nuclear program, the "direct dialogue between the United States and Iran" would be subject to the wait-and-see approach. There may be some tactical sense in that: The administration could preserve the international coalition it is trying to build while denying the shaky supreme leader the political boost that would come from direct dialogue with Washington.

Still, by now it ought to be clear that the best chance to protect what Mr. Obama calls "core U.S. security interests" lies in a victory for the Iranian opposition. That may look unlikely for now. But it is considerably more probable than a turn toward detente by those now engaged in murdering young women. There may not be much that can be done to help the opposition, though some tangible steps -- more money for broadcasting into the country, for example -- are readily available. But at the least, nothing should be done that would harm the cause of change. That is not just the moral course; it is the most pragmatic and realistic.