More than any other American military operation since the invasion of Iraq, the assassination yesterday of Maj. Gen. Qassim Suleimani, the head of Iran’s Qods Force of its Islamic Revolutionary Guards Corps, is a seismic event. The killings of Osama bin Laden and Abu Bakr al-Baghdadi, the leaders of al-Qaeda and the Islamic State, were certainly meaningful, but they were also largely symbolic, because their organizations had been mostly destroyed. Taking out the architect of the Islamic Republic’s decades-long active campaign of violence against the United States and its allies, especially Israel, represents a tectonic shift in Middle Eastern politics.

To see just how significant Mr. Suleimani’s death truly is, it helps to understand the geopolitical game he’d devoted his life to playing. In Lebanon, Mr. Soleimani built Lebanese Hezbollah into the powerful state within a state that we know today. A terrorist organization receiving its funds, arms and marching orders from Tehran, Hezbollah has a missile arsenal larger than that of most countries in the region. The group’s success has been astounding, helping to cement Iran’s influence not just in Lebanon but farther around the Arab world.

Building up on this successful experience, Mr. Suleimani spent the last decade replicating the Hezbollah model in Iraq, Syria and Yemen, propping up local militias with precision weapons and tactical know-how. In Syria, his forces have allied with Russia to prop up the regime of Bashar al-Assad, a project that, in practice, has meant driving over 10 million people from their homes and killing well over half a million. In Iraq, as we have seen in recent days, Mr. Suleimani’s militias ride roughshod over the legitimate state institutions. They rose to power, of course, after participating in an insurgency, of which he was the architect, against American and coalition forces. Hundreds of American soldiers lost their lives to the weapons that the Qods Force provided to its Iraqi proxies.

Mr. Suleimani built this empire of militias while betting that America would steer clear of an outright confrontation. This gambit certainly paid off under President Barack Obama, but it even seemed to be a safe bet under President Trump, despite his stated policy of “maximum pressure.” Mr. Trump was putting an economic squeeze on Iran, and popular protests in Iran, Iraq and Lebanon were adding to the pressure, but Mr. Suleimani assumed that, in the end, control of military assets would win the day. Mr. Trump, it seemed, feared getting sucked into a war. Washington, in short, lacked a ground game.