The successful operation against Qassem Soleimani, head of the Islamic Revolutionary Guard, is a stunning blow to international terrorism and a reassertion of American might. It will also test President Trump's Iran strategy. It is now Trump, not Ayatollah Khamenei, who has ascended a rung on the ladder of escalation by killing the military architect of Iran's Shiite empire. For years, Iran has set the rules. It was Iran that picked the time and place of confrontation. No more.

Reciprocity has been the key to understanding Donald Trump. Whether you are a media figure or a mullah, a prime minister or a pope, he will be good to you if you are good to him. Say something mean, though, or work against his interests, and he will respond in force. It won't be pretty. It won't be polite. There will be fallout. But you may think twice before crossing him again.

That has been the case with Iran. President Trump has conditioned his policies on Iranian behavior. When Iran spread its malign influence, Trump acted to check it. When Iran struck, Trump hit back: never disproportionately, never definitively. He left open the possibility of negotiations. He doesn't want to have the Greater Middle East—whether Libya, Syria, Iraq, Iran, Yemen, or Afghanistan—dominate his presidency the way it dominated those of Barack Obama and George W. Bush. America no longer needs Middle Eastern oil. Best keep the region on the back burner. Watch it so it doesn't boil over. Do not overcommit resources to this underdeveloped, war-torn, sectarian land.

The result was reciprocal antagonism. In 2018, Trump withdrew the United States from the Joint Comprehensive Plan of Action negotiated by his predecessor. He began jacking up sanctions. The Iranian economy turned to shambles. This "maximum pressure" campaign of economic warfare deprived the Iranian war machine of revenue and drove a wedge between the Iranian public and the Iranian government. Trump offered the opportunity to negotiate a new agreement. Iran refused.

And began to lash out. Last June, Iran's fingerprints were all over two oil tankers that exploded in the Persian Gulf. Trump tightened the screws. Iran downed a U.S. drone. Trump called off a military strike at the last minute and responded indirectly, with more sanctions, cyber attacks, and additional troop deployments to the region. Last September a drone fleet launched by Iranian proxies in Yemen devastated the Aramco oil facility in Abqaiq, Saudi Arabia. Trump responded as he had to previous incidents: nonviolently.

Iran slowly brought the region to a boil. First it hit boats, then drones, then the key infrastructure of a critical ally. On December 27 it went further. Members of the Kataib Hezbollah militia launched rockets at a U.S. installation near Kirkuk, Iraq. Four U.S. soldiers were wounded. An American contractor was killed.

Destroying physical objects merited economic sanctions and cyber intrusions. Ending lives required a lethal response. It arrived on December 29 when F-15s pounded five Kataib Hezbollah facilities across Iraq and Syria. At least 25 militiamen were killed. Then, when Kataib Hezbollah and other Iran-backed militias organized a mob to storm the U.S. embassy in Baghdad, setting fire to the grounds, America made a show of force and threatened severe reprisals. The angry crowd melted away.

The risk to the U.S. embassy—and the possibility of another Benghazi—must have angered Trump. "The game has changed," Secretary of Defense Esper said hours before the assassination of Soleimani at Baghdad airport. Indeed, it has. The decades-long gray-zone conflict between Iran and the United States manifested itself in subterfuge, terrorism, technological combat, financial chicanery, and proxy forces. Throughout it all, the two sides confronted each other directly only once: in the second half of Ronald Reagan's presidency. That is about to change.

Deterrence, says Fred Kagan of the American Enterprise Institute, is credibly holding at risk something your adversary holds dear. If the reports out of Iraq are true, President Trump has put at risk the entirety of the Iranian imperial enterprise even as his maximum pressure campaign strangles the Iranian economy and fosters domestic unrest. That will get the ayatollah's attention. And now the United States must prepare for his answer.

The bombs over Baghdad? That was Trump calling Khamenei's bluff. The game has changed. But it isn't over.