Last week, our military and intelligence services brought justice to Qassim Suleimani, Iran’s terror mastermind. President Trump ordered General Suleimani’s killing after months of attacks on Americans by Iran’s proxy forces in Iraq. These attacks culminated in a rocket strike that killed an American and wounded others, then the attempted storming of our embassy in Baghdad. The first attack crossed the red line drawn by the president last summer — that if Iran harmed an American, it would face severe consequences. The president meant what he said, as Mr. Suleimani learned the hard way.

Mr. Suleimani’s killing was justified, legal and strategically sound. But the president’s critics swarmed as usual. After the embassy attack, a Democratic senator declared that the president had “rendered America impotent.” Some Democrats then pivoted after the Suleimani strike, calling him “reckless” and “dangerous.” Those are the words of Senator Elizabeth Warren, who also described Mr. Suleimani — the leader of a State Department-designated Foreign Terrorist Organization plotting to kill American troops — as a “senior foreign military official.” Senator Bernie Sanders likened America’s killing of a terrorist on the battlefield to Vladimir Putin’s assassination of Russian political dissidents.

Some Democrats seem to feel a strange regret for the killing of a monster who specialized in killing Americans. The linguist his proxies killed on Dec. 27, Nawres Hamid, was merely his last victim out of more than 600 in Iraq since 2003. His forces have instigated attacks against our troops in Afghanistan. He plotted a (foiled) bombing in Washington, D.C., and attempted attacks on the soil of our European allies. He armed the terrorist group Hezbollah in Lebanon with rockets to pummel the Jewish state of Israel. And he was greeted moments before his death by a terrorist responsible for the bombing of our embassy in Kuwait in 1983.

Some of the president’s critics will concede that Mr. Suleimani was an evil man, but many complain his killing was unlawful. Wrong again. He was a United States-designated terrorist commander. As I have been briefed, he was plotting further attacks against Americans at the time of his death. The authority granted to the president under Article II of the Constitution provides ample legal basis for this strike. Furthermore, those who accept the constitutionality of the War Powers Act should recall that Congress’s 2001 and 2002 Authorizations for Use of Military Force very much remain in effect and clearly cover the Suleimani operation. This will be a relief to the Obama administration, which ordered hundreds of drone strikes using such a legal rationale.