Washington, DC - Tensions between the United States and Iran escalated dramatically last week after US President Donald Trump ordered the killing of top Iranian commander Qassem Soleimani in Iraq.

Iran promised to retaliate, and delivered on Wednesday, with missile attacks on two Iraqi bases housing US troops.

Following the attacks, Trump tweeted "all is good". He is set make a televised statement on Iran at 11amEST (16:00GMT) on Wednesday.

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Trump had previously threatened to "fully strike back" if Iran attacked any US citizen or site belonging to the US.

"The United States will quickly & fully strike back, & perhaps in a disproportionate manner," Trump tweeted over the weekend. He also said the US had identified 52 Iranian targets the US could strike, including Iranian cultural sites, which could constitute a war crime if carried out.

Angered that they were not consulted before the killing of Soleimani, House Democrats have planned a vote on a War Powers Resolution "to limit the president's military actions regarding Iran" later this week.

Iranian guards outside the United Nations office in Tehran hold pictures of the late Iranian Major-General Qassem Soleimani during a protest against his assassination with Iraqi militia commander Abu Mahdi al-Muhandis, in an air attack at Baghdad airport [West Asia News Agency/Nazanin Tabatabaee/Reuters] Iranian guards outside the United Nations office in Tehran hold pictures of the late Iranian Major-General Qassem Soleimani during a protest against his assassination with Iraqi militia commander Abu Mahdi al-Muhandis, in an air attack at Baghdad airport [West Asia News Agency/Nazanin Tabatabaee/Reuters]

The Trump administration has defended killing Soleimani, claiming the Iranian commander was planning an "imminent" attack on US forces in Iraq.

As the House prepares a vote, here is a look at the War Powers Resolution of 1973 and how it fits into the context of current tensions between Iran and the US:

What is the US War Powers Resolution of 1973?

Under the US Constitution, the authority to direct military action is divided between Congress and the president.

Congress has the power to declare war while the president, as commander-in-chief, has the power to use the military to defend the US.

From the beginning, presidents and Congress have disputed whether presidents have the constitutional authority to send forces into hostilities abroad without congressional authorisation, according to a history of the law prepared by the Congressional Research Service.

Such disputes gave way to the War Powers Resolution - also known as the War Powers Act - which was enacted in 1973 about the veto of then-President Richard Nixon. Congress wanted to constrain Nixon's ability to further commit US forces to the unpopular Vietnam War.

The law sets out procedures for Congress and the president to follow when sending US forces into hostilities.

It requires the president to consult "in every possible instance" with Congress before sending troops into actual or imminent hostilities and then submit a report to Congress any time US forces are deployed to conflicts or imminent conflict. The president must also report to Congress within 48 hours of US forces being introduced to hostilities.

Once forces are committed, the resolution requires withdrawal within 60 days (90 days in some circumstances) unless Congress authorises the use of force, or extends the time limit.

Why are war powers an issue now?

The air raid Trump ordered on Soleimani - a significant escalation - late last month in Iraq was undertaken without consultation or notice to Congress. It put US forces into imminent conflict, even a potential war with Iran that Congress has not authorised, Democrats argue.

The Trump administration has ratcheted up tensions with Iran since taking office in 2017. One of Trump's first decisions was to unilaterally withdraw from the 2015 United Nations-backed Iran nuclear deal, the JCPOA.

What will Congress do now?

The US House of Representatives is expected to vote this week on a War Powers Resolution seeking to limit Trump's ability to act against Iran, House Speaker Nancy Pelosi announced on Sunday.

"Last week, the Trump administration conducted a provocative and disproportionate military air strike targeting high-level Iranian military officials," Pelosi, a Democrat, said in a statement.

"This action endangered our service members, diplomats and others by risking a serious escalation of tensions with Iran," she said.

In the US Senate, where the resolution is expected to face an uphill battle, Democratic Senator Tim Kaine said he will demand a vote from the Republican majority.

Protesters in Iran burn a US flag during a demonstration on the US air attack in Iraq that killed Soleimani [Vahid Salemi/AP Photo]

"We should not go to war with Iran based on a presidential decision with no consultation of Congress," Kaine told MSNBC television last week.

"We can't let the president make this decision on his own," Kaine said. Even if a resolution passes in the House and Senate, it would likely face a Trump veto.

The Trump administration will give a classified briefing to congressional leaders on Tuesday. The full House and Senate will receive separate classified briefings on Wednesday.

When has Congress authorised war?

After the September 11, 2001 al-Qaeda attacks in the US, Congress authorised the use of military force in Afghanistan and globally against al-Qaeda.

In 2002, Congress gave President George W Bush authority to invade Iraq on the pretext - later proved false - that then-Iraqi President Saddam Hussein was building weapons of mass destruction.

Beginning in June 2014, former President Barack Obama notified Congress repeatedly of strikes and deployments against the Islamic State of Iraq and the Levant (ISIL or ISIS) in Syria.

Obama and Trump claimed authority from the 2001 and 2002 authorisations by Congress for the military campaign against ISIL.

In recent years, there has been a continuing debate in Congress on whether the 2001 and 2002 authorisations should be replaced with a new, more narrowly tailored congressional authority for US military action.

The US currently has 60,000 troops in the Middle East and North Africa, according to the Pentagon.