Last week, the United States launched an air strike on a Baghdad airport, killing General Qassem Soleimani - one of Iran's most senior commanders.
General Soleimani had been commander of the Islamic Revolutionary Guard Corps and as chief of the elite Quds Force unit, was responsible for the Islamic Republic's military campaigns abroad.
He is thought to have played a role in the deaths of thousands of civilians and western military personnel.
The US said the Iranian's death was ordered because he was "actively developing plans to attack American diplomats and service members in Iraq and throughout the region".
Iran retaliated by attacking military bases in Iraq, containing US and UK personnel.
Why did the United States kill General Soleimani?
After the news of the air strike which killed General Soleimani in Baghdad, US President Donald Trump echoed the stance of the Pentagon and said the top Iranian general "should have been taken out many years ago".
Mr Trump also said Gen Soleimani was "plotting to kill" many Americans and that he was "directly and indirectly responsible for the death of millions of people".
Boris Johnson told the House of Commons on Wednesday that General Soleimani had supplied "improvised explosive devices to terrorists, which I'm afraid killed and maimed British troops".
The Prime Minister added: "That man had the blood of British troops on his hands."
American officials say they have intelligence which shows General Qassem Soleimani "was planning, coordinating, and synchronising significant combat operations against US military forces".
During a press conference on Monday, chairman of the Joint Chiefs of Staff General Mark Milley said he had access to intelligence files showing Gen Soleimani had been plotting against the United States.
"I'll stand by the intelligence I saw, that was compelling, it was imminent, and it was very, very clear in scale, scope," he said.
"You've got a very long history here of a guy.
"We know his history. Importantly we knew his future."
US Defense Secretary Mark Esper called Soleimani "a terrorist, a leader of a terrorist organisation who's been killing and attacking Americans for 20 some years".
"The blood is on his hands," he added.
"He was planning attacks on American forces."
"He was there on the ground with the leader of Kataeb Hezbollah, met him on the ground at the airplane, welcomed him so they can further coordinate attacks," Mr Esper said.
During a press briefing on Wednesday, US Foreign Secretary Mike Pompeo said they had "multiple pieces of information" prior to launching the attack on General Soleimani.
"We could see clearly that not only had Soleimani done all of the things that we have recounted... a massacre in Syria, enormous destruction of countries like Lebanon and Iraq," Mr Pompeo said.
"And then we’d watch as he was continuing the terror campaign in the region."
The US Foreign Secretary added: "Then you, in addition to that, have what we could clearly see were continuing efforts on behalf of this terrorist to build out a network of campaign activities that were going to lead potentially to the death of many more Americans.
"It was the right decision. We got it right."
Following the killing of Gen Soleimani, Iraqi parliament voted to expel all foreign forces from the country.
The United States has denied that American troops will be pulled out of Iraq, after an unsigned draft letter from a senior military officer appeared to suggest plans for withdrawal were underway.
US and coalition training missions on the ground in Iraq, however, have been suspended.
Cover image: a shot of General Qasem Soleimani in June 2018 (Picture: PA).