WASHINGTON (AFP) - US President Barack Obama will steel Americans for a prolonged battle against the Islamic State (IS) on Wednesday, despite devoting much of his presidency to avoiding sapping new entanglements in the Middle East.
His hand forced by the radical group's sudden rise in Syria and Iraq, Obama will use a prime time address to argue that targeted military might and an international coalition can defeat IS, before it poses a threat to the US homeland.
But mindful of avoiding what he believes are the mistakes of the last decade, Obama will assure millions of television viewers that he will not send conventional ground troops back to Iraq to fight a group that has beheaded two US journalists.
It remained unclear whether Obama would also use the address on the ceremonial state floor of the White House - where he announced the success of a US operation to kill Osama bin Laden - to signal an expansion of US airstrikes against IS from Iraq into Syria.
The speech will also lack a definitive timeline for US operations against IS, after several reports cited senior officials as saying they could outlast Obama's presidency, which ends in January 2017.
"I think the American people need to expect that this is something that will require a sustained commitment," said White House spokesman Josh Earnest.
The address will come at a poignant time - on the eve of the 13th anniversary of the September 11 attacks, when Islamic radicalism on a mass scale scorched the US homeland for the first time, and drew America into exhausting overseas wars and a still unending anti-terror campaign.
Earnest said Obama would "talk about the risks that the United States faces, and he'll talk about the strategy that he has put together to confront those risks, to mitigate them, and ultimately to degrade and destroy" IS, he said.
But many details of the plan Obama will outline, as he enlists the symbolic weight of a national address at 9.00 pm (0100 GMT Thursday) remain unclear.
But White House aides say it will be anchored on the cautious hope that Iraq's new unity government will prove more inclusive than ex-prime minister Nuri al-Maliki who it blamed for fanning sectarianism that eased Islamic State's rise.
The president briefed senior congressional leaders on his plan on Tuesday, and an aide to House Speaker John Boehner stirred speculation that he could order US troops back to Iraq on a mission strictly limited to training Iraqi troops and to call in air strikes against IS forces.
"The speaker stated he would support the president if he chose to deploy the military to help train and play an advisory role for the Iraqi security forces and assist with the lethal targeting" of IS leadership, the aide said. There was no comment from the White House on whether Obama was considering such an idea, after aborting plans for a small contingent of American soldiers to stay on in Iraq after 2011, following a disagreement with the Maliki government.
Obama, who sees ending wars in Afghanistan and Iraq as a centerpiece of his legacy, is also under pressure to announce stepped-up support for moderate rebels of the Free Syrian Army, despite his antipathy to intervening in Syria's vicious civil war.
Washington wants to ensure that President Bashar al-Assad, who it regards as a war criminal, does not benefit from any power vacuum left in the event that US military action degrades IS. A White House official said that Obama told congressional leaders on Tuesday that he did not need fresh permission for military action he is planning to take against IS. Senior lawmakers are divided on whether existing authorizations to act against terror groups cover an expanded campaign against the jihadist group.
A senior US official however told AFP that Obama told the bipartisan leadership of the House and the Senate that he did need lawmakers to vote on intensifying US training and equipping of moderate Syrian rebels. Obama previously asked for such authorisation in May, and wants action before Congress leaves town within weeks, ahead of midterm elections in November.
The speech will represent a chance for Obama to redress criticism that he has been slow to respond to IS, amid fears fighters armed with Western passports could hit US targets.
Obama dented his credibility two weeks ago when he admitted he did not yet have a strategy to take on IS in Syria. The president started the work of creating an international coalition to take on IS at the Nato summit last week.
Defence Secretary Chuck Hagel has been in the Middle East, and Secretary of State John Kerry arrives in the region on Wednesday to accelerate Washington's efforts.
Public opinion in the United States meanwhile appears to be shifting in favour of a limited intervention in Syria. A Wall Street Journal/NBC News poll published on Tuesday found two-thirds of those asked favoured taking on IS.