Does Obama need Congress to attack Islamic State?
President Obama says he has the authority to pursue Islamic State in the way he wants. Not everyone agrees.
Mr Obama is now going after Islamic State or Isil, as he calls the group, with a vengeance.
"We will degrade and ultimately destroy Isil," he said in an address at the White House on Wednesday. He said US forces would pursue the militants in Iraq - and now in Syria, too.
He asserts he has the authority to use US force in this way and does not need the approval of the US Congress. His advisers say they already have authorisation.
Several days after al-Qaeda struck the US in 2001, lawmakers passed an authorisation for use of military force (AUMF). It does not specifically name al-Qaeda - but is directed at the organisation.
Hunting down al-Qaeda
The resolution states: "The president is authorised to use all necessary and appropriate force against those nations, organisations or persons he determines planned, authorised, committed, or aided the terrorist attacks."
With this resolution, President George W Bush felt he could pursue the people who carried out the attacks, whether they were in Afghanistan or in other parts of the world.
Before Mr Obama's speech, a senior administration official told reporters the president could rely on the 2001 resolution "as statutory authority for the military air strike operations he is directing against Isil".
The 2001 resolution was part of the "security apparatus", as former Vice-President Dick Cheney described it, created in an effort to combat al-Qaeda and allied groups. ("I was one of its architects," said Mr Cheney in a speech at the American Enterprise Institute, a conservative think tank, on Wednesday.)
Marc Thiessen, a former speechwriter for Mr Bush, said Mr Obama can - and should - use the work of Bush administration officials in his campaign against Islamic State.
"There was a legal architecture that allowed the US to fight al-Qaeda in Iraq," Mr Thiessen said. "The president is fighting the same group with a different name."
Mr Thiessen added: "If you're fighting an enemy in Iraq and they run across the border into Syria, you don't need additional authorisation to go after them - just like we didn't need additional authorisation when the Germans went into France."
Shocked by grisly killings
Last year Mr Obama planned to ask lawmakers to authorise military action in Syria. He wanted to retaliate against President Bashar al-Assad for the alleged use of chemical weapons.
But it seemed unlikely Congress would support the president, and Mr Obama dropped his plans.
Today things are different.
Mr Obama is not asking Congress for direct approval for military action. Instead he wants lawmakers to approve $500m (£386m) for rebels in Syria combating Islamic State.
Mr Obama has a sympathetic audience on Capitol Hill. The Republican Speaker of the House, John Boehner, has offered his support. "We ought to give the president what he's asking for," he told reporters.
Like other Americans, lawmakers were shocked by the videos released by Islamic State, showing the brutal executions of journalists James Foley and Steven Sotloff.
Most Americans now say using military force against Islamic State is in the nation's interest, according to an NBC News/Wall Street Journal poll.
Going it alone
US presidents have frequently used military force without congressional approval
1983: President Ronald Reagan sent troops to Grenada
1986: Mr Reagan authorised air strikes in Libya
1998: President Bill Clinton shot missiles to Sudan and Afghanistan
1999: Mr Clinton sent forces to Kosovo on a NATO mission
2011: Mr Obama authorised air strikes in Libya
Source: Wall Street Journal, Congressional Research Service
Some legal experts believe the president is wrong in their assessment of the AUMF.
Deborah Pearlstein, a professor at Cardozo School of Law in New York, said Islamic State is no longer part of al-Qaeda. "They're enemies," she said. "By attacking today's [IS] we are in no sense attacking al-Qaeda."
The use of presidential power
When experts weigh in on this issue, they sound authoritative. That is how the president sounds this week. So do the people who support his decision - and those who oppose him.
Yet Mr Obama's decision to use air strikes in Syria is controversial. The laws that govern his deployment of force are murky.
"It's not so clearly, neatly and authoritatively defined - and tends to be decided through a wrestling match between Congress and the president," said Matthew Waxman, who was a senior official in the defence and state departments during the Bush administration.
Still, presidents hardly ever get in trouble for using the military. Chances are Mr Obama will face little if any, repercussions for authorising the strikes.
Right or wrong, he is acting presidential.