U.S. officials tracked Iranian threats against President Trump before ceasefire ended

Iran's president has said in recent months that the government sees vengeance against U.S. officials as a priority.

Historical context The Soleimani killing accelerated a lifting of restraints on foreign assassinations, and the taboo on targeting and killing foreign leaders with U.S. military assets has been more or less lifted, said a political professor at George Washington University.

If the United States sets the example of how to conduct international relations, and it is using assassination of foreign leaders as a political weapon, it's only logical that other countries will be more inclined to also engage in assassinations, the professor added.

Returning from a NATO summit in Turkey on Wednesday, Trump was forced to switch back to an old model of Air Force One equipped with specialized defensive technologies from a new plane given as a gift by Qatar, after the Secret Service warned of potential threats to the aircraft from Iran.

They want to take out the U.S. leader, Trump told reporters aboard the plane. I'm on whatever list. I saw this morning I'm on every single one of their lists. The threat has remained on his mind in the days since. In a subsequent social media post Friday night, he warned of a catastrophic response he instructed the administration to pursue in the event Tehran succeeds.

The United States had a decades-old prohibition against assassinating foreign leaders before Trump's presidency, codified in an executive order signed by President Ford in 1976 over concerns of a CIA plot to kill Fidel Castro. The policy was only strengthened further by subsequent administrations, fearing a new international standard for targeted killings could result in catastrophic consequences in the halls of Washington.

Under the Obama administration, an international coalition targeting the Libyan regime of Muammar Gaddafi during the country's 2011 civil war struck his fleeing convoy, leading to his capture and killing by rebel fighters. But experts say Trump's explicit targeting of Soleimani and the supreme leader, and his public celebration of their deaths, marks a new paradigm.