Iran is challenging the might-is-right cornerstone of Donald Trump’s foreign policy by refusing to bow to the US president’s demands and fighting a war that within hours expanded across the Middle East.

In doing so, Iran is going where no other country, including Venezuela and NATO ally Denmark has been willing to go when threatened with military force if they did not accept Mr. Trump’s demands.

Iran was betting that Mr. Trump would want a quick strike against Iran that would not entangle the United States in a protracted conflict and potentially force it to put boots on the ground.

It was a miscalculation. Nevertheless, it was a risk Iran willingly shouldered.

Joined by Israel in the attack on Iran, Mr. Trump and Israeli Prime Minister Binyamin Netanyahu have suggested that regime change was the attack’s goal. Mr. Trump has acknowledged that achieving that goal could involve protracted hostilities in which US troops may be killed.

Mr. Khamenei was killed on the first day of the US and Israeli strikes. So were other officials, including the commander of Iran's Islamic Revolutionary Guard Corps, Gen. Mohammad Pakpour, who was appointed by Mr. Khamenei.

Even so, it would be premature for the United States and Israel to declare victory. Mr. Khamenei’s death does not mean the collapse of the regime.