A significant Iranian retaliation is to be expected, and this could lead to a cycle of action and reaction that could bring the two countries ever closer to an all-out conflict
Iranian military commander Qasem Soleimani was killed by the US in an airstrike in Iraq yesterday.
Soleimani's killing is now fuelling tensions between Washington and Tehran.
To explain the actions, BBC defence and diplomatic correspondent Jonathan Marcus answers some of the public's following questions.
How likely is this to escalate to WW3? - Lewis Alcott
While some have described the killing of Soleimani as "a declaration of war" by the United States against Iran, it is important both not to overstate nor to understate the significance of the moment.
This will not prompt World War Three. The key actors who might be involved in such a conflict, for example Russia and China, are not significant players in this drama.
But this could become a defining moment for the Middle East and for Washington's role in it. A significant Iranian retaliation is to be expected, and this could lead to a cycle of action and reaction that could bring the two countries ever closer to an all-out conflict.
Iran's response might be against US military interests in the region but equally it could be against any US-related target that Iran thinks vulnerable.
Is it legal to kill someone like this under international law? - Eamonn Donaghy
The US would argue that Soleimani was responsible for unprovoked attacks on American forces in Iraq. Those forces were there at the request of the current Iraqi government.
Soleimani was a man whom Washington believed already had the blood of many US personnel on his hands. Meanwhile, the Quds organisation he headed was seen by the US as a terrorist organisation. So his killing may follow a US legal narrative.
But the noted international legal scholar, Notre Dame Law School Prof Mary Ellen O'Connell, has this view of the legal implications:
"Pre-emptive self-defence is never a legal justification for assassination. Nothing is. The relevant law is the United Nations Charter, which defines self-defence as a right to respond to an actual and significant armed attack," she said.
"The use of a drone to kill Iranian Gen Qasem Soleimani in Baghdad was not in response to an armed attack on the United States. Iran has not attacked the sovereign territory of the United States," she said.
"In this case, the United States has not only committed an extrajudicial killing, it has carried out an unlawful attack within Iraq."
Where does the UN stand on these killings? - Sara
Beyond the stated views of individual representatives, it is hard to say what the UN view is, since there really is no such thing.
Does one, for example, mean the considered view of the UN Security Council? That is likely to be divided and unable to reach consensus.
Secretary General Antonio Guterres said he was deeply concerned by the rise in tensions in the Middle East.
"This is a moment in which leaders must exercise maximum restraint. The world cannot afford another war in the Gulf," his spokesman, Farhan Haq, said in a statement.
Was this ordered to deflect President Trump from the impeachment trial? - Martin Gallagher
It is easy to make these kinds of charges but, while domestic political considerations always matter - particularly during an election year for President Trump - this decision would be a product of two factors: opportunity and circumstance.
Why kill Qasem Soleimani now and what happens next?
The context seems to be the escalating attacks on US facilities in Iraq, along with vague assertions by the Pentagon about future attacks in the making.
And the opportunity presented itself - a further demonstration of the accuracy and reach of US intelligence - which, while far from infallible, is a factor that the Iranians will need to contend with in deciding upon any response.
In an election year, President Trump's main concern is to avoid the loss of US lives in the region.
This dramatic strike seems in some way out of character for a president who, while talking tough, has been characterised by remarkable caution in terms of actions.
Is there any danger of Iran pursuing a nuclear response? Does it have a nuclear capability? - Harry Rickman
No. Iran does not have a nuclear weapons programme as such, though it retains many elements that could contribute to such a programme and the know-how to proceed with one.
Iran has always insisted that it does not want the bomb. But could growing frustration with Washington persuade Iran to throw off all constraints and essentially abandon its nuclear agreement with the international community altogether? That is a possibility.
The Trump administration has already abandoned the so-called JCPOA agreement or Iran nuclear deal - many analysts might say recklessly - raising the pressure on Tehran but without any clear diplomatic "off ramp" to contain the tension.
What was Gen Soleimani doing in Iraq? What does the Iraqi government say about this? - Tom
It's not clear what precisely the general's business was in Iraq. But Iran supports a variety of influential Shia militia groups there and the man who was killed alongside him, Abu Mahdi al-Muhandis - was the leader of Kataib Hezbollah (the group said to be responsible for recent rocket attacks on US bases) and the deputy commander of a coalition of pro-Iranian militias in Iraq.
The Iraqi government has been put in a very difficult position, especially since the attack came on its soil. It is an ally both of Iran and of the US, and US troops remain in Iraq to assist in the broader struggle against the Islamic State (IS) group.
The Iraqi authorities were already embarrassed by the militia attacks against bases housing US facilities. The Iraqi government both condemned recent US reprisals against this militia group while insisting to the Americans that they would do more to protect the bases.
The Iraqi prime minister's office condemned the killing of Soleimani, and described him and the militia leader killed alongside him as "martyrs" behind the "great victories against IS". The Iraqi government also insists that the US acted way beyond the terms of the agreements under which it operates in the country.
What is the role of the US and Iran in Iraq? - Kakinga Moses
Iran is a close ally of the Shia-led government in Iraq. It is also a significant player in the country in its own right, working through the militia groups mentioned above. The US has some 5,000 troops in Iraq, training and mentoring the Iraqi military in its effort to defeat the remaining IS elements.
Essentially these two outside players - the US and Iran - have been maneuvering against each other in Iraq.
One big question now is will a moment of crisis come that makes a continued US presence in the country untenable?