Partisans may trade blame, but whether because of the 2003 United States-led invasion of Iraq or the withdrawal of American troops from Iraq in 2011, Iran’s efforts to destabilize the Middle East have accelerated. The Syrian civil war only contributed to the perfect storm. Tehran is now aggressively interfering in countries from Afghanistan to the Eastern Mediterranean, and perhaps even farther. And while some place blame on Sunni governments such as Saudi Arabia for providing Iran with opportunities to meddle, Sunni repression hardly excuses Iran’s transfer of weapons to Yemen, Lebanon and others in violation of United Nations Security Council resolutions or Tehran’s broader efforts to undermine American allies like Saudi Arabia and Bahrain.

Strangely, however, Mr. Trump has done little to push back on Iranian expansionism. The United States provides cursory support for operations by the Saudis and United Arab Emirates against Iranian-backed forces in Yemen. And for most of this year, the administration has been funneling financial aid to the Lebanese armed forces, which in turn have been working hand in hand with Iran’s most powerful proxy, Hezbollah, on the Lebanon-Syria border. While the administration has offered inconsistent and lackluster support for the Arab nations challenging Qatar’s support for extremists, it has largely ignored Iran’s growing influence in both Qatar and Oman.

Mr. Trump’s oddest capitulations to Iran are in Iraq and Syria. Rather than seize opportunities to push back on Iranian power while pursuing the annihilation of the Islamic State, Team Trump has largely embraced Obama-era nonpolicies in both states. In Iraq, the central government is desperate for assistance to rein in Iranian-backed militias that may have contributed to the fight against the Islamic State but now threaten the country’s stability. If the goal is to prevent a repeat in Iraq of Hezbollah’s slow strangulation of Lebanon, the time is now.

Similarly in Syria, despite early hints that he was poised to take on the Tehran-Moscow-Damascus triumvirate, Mr. Trump has been almost supine, to the point of ignoring attacks on American-backed forces. The White House has even shrugged off news that Iran has opened another front against Israel in the Syrian Golan Heights.

If rolling back and diminishing Iranian power is the priority Mr. Trump insists it is, simply dumping the nuclear agreement in Congress’s lap may be the worst possible option. That would be politically easy, but it won’t get the job done.