From A Lawless Political Assassination, by Judge Andrew P. Napolitano:
The Constitution provides only two means for the federal government to kill a human being. The first is pursuant to a declaration of war, which only Congress can do. That permits the president to use the military to kill the troops of the government of the country against which war has been declared. Congress has not declared war on Iran. The second way that the Constitution permits federal government killings is pursuant to due process. That means that the person to be killed is lawfully in custody, has been properly charged, lawfully tried and fairly convicted of a capital crime, and the conviction has been upheld on appeal.
[…]
Think about it. If the American president can kill an Iranian government official in Iraq because of fear of what he might do — without a declaration of war or any legal process — can the Chinese president kill a Mexican government official visiting in Texas or an American intelligence agent encouraging revolution in Venezuela for fear of what they might do? This is not a fanciful or academic argument. It not only goes to the fidelity to the rule of law that we require of our leaders in order to maintain personal liberty and limited government, it also goes to our safety. We have laws to prevent wanton killings, lest killers turn on us.
In contrast, Attorney Dershowitz Says Targeting Soleimani Was Constitutional, But Impeachment Is Not:
“I am an expert on the use of targeted killings and a strong supporter of targeted killings of terrorists and ongoing terrorist situations. But it’s not something the president discussed with me. I wrote a piece for The Wall Street Journal yesterday, and I’ve been talking about what I believe is the strong case for the legality. I don’t take a position, particularly on the long-term wisdom of the action. But I think the legality is not even a close question. I think it was more legal, if anything, than the killing of Osama bin Laden, because the Osama bin Laden killing was not preventive. It was vengeance. It was getting even with a massive criminal that was justified, but it was justified on different grounds. The Soleimani case is a much stronger case for preemptive or preventive targeted killing.”
On whether Trump made the right decision by ordering the killing of Soleimani “I think that’s reasonable. People could disagree about that. But I don’t think anybody should conflate the policy arguments with the legal arguments, and many people do that. Something could be illegal and good policy and something could be legal and bad policy.” [Here & Now]
U.S. Sen. Joe Lieberman in the WSJ notes that former President Barack Obama ordered drone strikes and “[h]e did so without specific congressional authorization, and without significant Democratic opposition.”