During a debate in New Hampshire, Senator Bernie Sanders said that such actions opened the door “to international anarchy.”
He said the United States needs to “sit down and work out our differences through debate and discussion at the UN, not through war and more war and expenditures of trillions of dollars and the loss of God knows how many lives.”
“What we have got to do, which [President Donald] Trump does not understand, is strengthen the State Department in our diplomatic capabilities, not just the military,” the Vermont senator said.
Former vice president Joe Biden said he would not have ordered the drone strike if he had been in the White House.
"The reason I wouldn’t have ordered the strike is there isn’t any evidence yet of an imminent strike that was going to come from him," Biden said.
Former South Bend, Indiana, mayor Pete Buttigieg said, “In the situation that we saw with President Trump’s decision, there is no evidence that [assassinating Soleimani] made our country safer.”
The Trump administration has said Soleimani was planning attacks against Americans, though critics have said there is no information to back up that claim.
The assassination brought the two nations to the brink, with Iran vowing revenge and the countries seeming poised for possible war. But the situation has since cooled after Iran launched a missile attack on US forces based in Iraq days later, injuring dozens of American soldiers.
The Hill and axios.com contributed to this story.