How to give diplomacy a chance with Iran

America can engage militarily at any time. No doubt the Pentagon has proposed several courses of action, ranging from B-2 strikes on Iran’s Fordow enrichment site using 30,000-pound bunker-buster bombs to a full-fledged joint military campaign with Israel to take out Iran’s potential retaliatory tools, disable its defenses, and send in special forces to confirm nuclear sites have been destroyed. These variations are part of the track-one option.

Mr. Trump, however, has the leverage to achieve a track-two approach: a diplomatic solution that would defang the regime. Mr. Trump could require Iran to stop all its nuclear efforts and give up uranium enrichment permanently, renounce its aims to destroy Israel and the U.S., end its hegemonic aspirations, and cease supporting terrorism. In return, the U.S. wouldn’t destroy Iran with military force.

The U.S., in conjunction with the International Atomic Energy Agency, would inspect and decommission all existing Iranian nuclear sites and implement regular inspections to prevent Iran from ever again seeking nuclear weapons. When Iran wants nuclear power for civilian infrastructure, it can purchase U.S. reactors and externally enriched uranium with appropriate safeguards.

Mr. Trump need not rush to decide whether to strike Iran. With each passing day, Tehran expends its arsenal and is weakened by Israel’s air attacks. This gives Mr. Trump strong footing and rare bargaining power. If the U.S. joins Israel in attacking Iran, then the leverage against the ayatollah and his government will be even greater. Engaging militarily—pursuing some range of track-one tactics—would not end the opportunity for a track-two diplomatic path but would only enhance it.

The critical question the Trump administration must ask: What is our desired end goal? If it is to end the threat of a theocratic, nuclear-armed terrorist state that is determined to destroy other nations, then we must consider how that could be best achieved. We must also be realistic and consider which is more likely to produce another failed state—a diplomatic solution or a military campaign.

We have seen the end results of four major U.S. military campaigns since 1999. Three were military successes but resulted in costly failures in the aftermath because of a lack of forethought and planning. After the Taliban’s collapse in 2001, the U.S. attempted to help Afghanistan establish a new government. Efforts to stabilize the country failed, and the Taliban rules again. In 2003, after capturing Saddam Hussein, the U.S. failed to plan seriously for Iraq’s future. As a result, Iraq has become a magnet for terrorism and a major source of migrants. And while the U.S. fought a long campaign against ISIS in Syria and Iraq, the potential for a resurgence of terrorism remains.

Only in 1999, when U.S. and North Atlantic Treaty Organization forces engaged in a 78-day air campaign to end ethnic cleansing in Kosovo, was the end state successful. There are a few reasons for this: The country already had a democratically organized and popular political party, led by Ibrahim Rugova, as well as an armed resistance army. NATO had already planned and agreed to secure the country and help end the fighting. A combination of NATO’s air campaign and diplomatic pressure from Finnish and Russian leaders led Slobodan Milosevic, then president of Yugoslavia, to realize that he had no alternative but to accept NATO’s terms and end the ethnic cleansing.

Iran today is also clearly losing, so the U.S. shouldn’t take Iran’s threats and angry bluster to mean the door on diplomacy is closed. Israel has the momentum to sustain its air campaign for weeks while Iran exhausts its inventory of missiles, depletes its military and industrial base and loses more of its command and control. Time isn’t on Iran’s side; the ayatollah and his remaining leadership know this. Air power alone, however, can’t guarantee the end of Iran’s nuclear ambitions. To rid Iran of its nuclear capabilities through military action would require a level of in-country military engagement that Americans have come to abhor.

History shows us that successful diplomacy starts with an achievable endgame. “Regime change" is a whispered goal. But absent a thoughtful, well-planned effort led by Israel and the U.S., Iran could end up as another failed state—a hotbed for further terrorism, insurgency and migrant outflows. If the West acts as a bystander, it’s possible that the Iranian people would overthrow the government. But then what? What guarantee do we have that a successor regime won’t relaunch Iran’s nuclear ambitions?

The most rational endgame is to give the mullahs a choice: Give up uranium enrichment and the nuclear ambitions it enables. Give up the proxy terror war against Israel and its supporters. In return, escape more-severe military attacks and the crippling sanctions that have decimated the Iranian economy. The U.S. should allow the mullahs to survive but should leave government to the Iranian people. Enable Iranians to engage in open and internationally supervised elections, with the hope of Persia’s return to peace and prosperity.

The U.S. has a rare opportunity to combine the leverage of a military campaign with strategic diplomacy to force Iran’s remaining leadership to confront their real choice: likely being overthrown and killed by their own people, or giving up their aggressive ambitions and renouncing their hold on government. If they choose wrongly, they will reap the consequences.

The power is in our hands. Do we have the wisdom, gained by painful experience, to achieve a more peaceful Middle East?

Mr. Clark, a retired U.S. Army general, served as NATO’s supreme allied commander Europe, 1997-2000.