FIVE DAYS of street protests in cities across Iran have underlined the fundamental weakness of a regime sometimes portrayed in Washington as a regional juggernaut. Despite the lifting of most Western economic sanctions after 2015, the Islamic republic has been unable to satisfy the expectations of everyday Iranians, who see the country's resources squandered on corruption and foreign military adventures by clerics who deny basic freedoms. Protests that began in one city over rising food prices quickly mushroomed into a nationwide uprising directed squarely at the rule of Supreme Leader Ali Khamenei.
The popular demand for change is justified and deserves international support. President Trump has been right to tweet his backing for the demonstrators; European leaders, who have been far more cautious, should speak up. At the same time, it's important to mind the lessons of history, which suggest that the odds that the protesters will trigger a revolution are long. The Khamenei regime has proved ruthlessly adept at putting down previous opposition movements, most recently in 2009, and still has abundant repressive resources at its disposal.
The new unrest so far differs considerably from that of 2009 in ways that probably advantage the regime. It lacks leaders or a clear agenda; the "Green Movement" grew out of protests following a presidential election that united liberal forces. The current demonstrations started in provincial city Mashhad and may have been initially encouraged by conservative forces opposed to the government of President Hassan Rouhani. While they spread to dozens of small cities and towns, Tehran, the center of the 2009 movement, appears to have been less galvanized.
This uprising also appears to be more violent. Authorities and independent observers have reported attacks on government facilities and even military bases in some cities; a dozen people were reported killed as of Monday. That could give the regime a pretext for a bloody crackdown, using the Revolutionary Guard Corps or the Shiite militia forces it has marshaled for wars in Iraq and Syria.
So far the guards appear to be waiting on the sidelines while Mr. Rouhani, a relative moderate in the complex Iranian political system, offers conciliatory messages. On Sunday he recognized that the demonstrators had legitimate grievances and nominally accepted their right to protest. The Trump administration and other Western governments should aim to hold him to those words through diplomacy and the threat of sanctions in the event of more bloodshed. Western leaders should also do what they can to support peaceful protests, including by looking for ways to help Iranians communicate with one another as the regime restricts the Internet.
At the same time, Mr. Trump should avoid acts that would undercut the protests and empower the regime's hard-liners. Foremost among these would be a renunciation of the 2015 nuclear accord. That would divide the United States from European governments when they should be coordinating their response to the uprising, and it would give the regime an external threat against which to rally. Reform of the nuclear accord can wait. Now is the time for Mr. Trump to focus on supporting the people of Iran.