President Barack Obama has ordered the US into a broad military campaign to "degrade and ultimately destroy" Islamic State (IS) militants in two volatile Middle East nations, authorising air strikes inside Syria for the first time as well as an expansion of strikes in Iraq.
In a major reversal, Mr Obama also announced in a televised addressed to the nation that he was dispatching nearly 500 more US troops to Iraq to assist that country's besieged security forces.
And he called on Congress to authorise a programme to train and arm rebels in Syria who are fighting both the IS group and Syrian president Bashar Assad.
Saudi Arabia, a crucial US ally in the Middle East, offered to host the training missions, part of Mr Obama's effort to persuade other nations to join with the US in confronting the militants.
"This is not our fight alone," Mr Obama declared. "American power can make a decisive difference, but we cannot do for Iraqis what they must do for themselves, nor can we take the place of Arab partners in securing their region."
"Our objective is clear: We will degrade and ultimately destroy Isil (IS) through a comprehensive and sustained counter-terrorism strategy."
The president adamantly ruled out the prospect of putting American troops in combat roles on the ground in Iraq or Syria.
Even so, Mr Obama's plans amount to a striking shift for a president who rose to political prominence in part because of his early opposition to the Iraq war.
While in office, he's steadfastly sought to wind down American military campaigns in the Middle East and avoid new wars - particularly in Syria, a country where the chaos of a lengthy civil war has given the IS space to thrive and move freely across the border with Iraq.
Speaking on the eve of the anniversary of the September 11, 2001, attacks, Mr Obama's plans also amounted to an admission that years of American-led war in the Middle East have not quelled the terror threat emanating from the region.
While administration officials have said they are not aware of a credible threat of a potential attack by the militants in the US, they say the IS group poses risks to Americans and interests in the region.
Officials are also concerned about the prospect that Westerners, including Americans, who have joined the militant group could return to their home countries to launch attacks.
In recent weeks, the militants have released videos depicting the beheading of two American journalists in Syria.
The violent images appear to have had an impact on a formerly war-weary public, with multiple polls in recent days showing that the majority of Americans support air strikes in both Iraq and Syria.
The US began launching limited air strikes against IS targets in Iraq earlier this summer. But officials said Mr Obama was waiting for Iraq to form a new government - a step it took on Tuesday - before broadening the effort.
Officials said strikes in Iraq would now be wide-ranging and extend into Syria. Mr Obama plans to proceed with those actions without seeking new authorisation from the US Congress.
Instead, officials said Mr Obama will act under a use of force authorisation Congress passed in the days after the September 11 attacks to give then-president George W Bush the ability to go after those who perpetrated the terror attacks.
Mr Obama has previously called for that authorisation to be repealed, he has also used the measure as a rationale to take strikes against terror targets in Yemen and Somalia.
Officials compared the new US mission in Iraq and Syria to the actions in Yemen and Somalia, campaigns that have gone on for years.
Mr Obama is seeking authorisation from Congress for a Pentagon-led effort to train and arm more moderate elements of the Syrian opposition. Ahead of Mr Obama's remarks, congressional leaders grappled with whether to support that request and if so, how to get such a measure through the fractured legislature before the November elections.
The White House wants Congress to include the authorisation in a temporary funding measure they are expected to vote on before adjourning later this month. Republicans made no commitment to support the request and the House GOP has so far not included the measure in the funding legislation.
A spokesman for US Senate majority leader Harry Reid said the Nevada Democrat might opt to seek separate legislation to authorise the president's request.
While the CIA currently runs a small programme to arm the rebels, the new programme would be more robust.
Mr Obama asked Congress earlier this year to approve a 500 million US dollars (£308 million) program to expand the effort and put it under Pentagon control, but the request stalled on Capitol Hill.
Some of the president's own advisers, including former secretary of state Hillary Rodham Clinton, pressed him to arm the rebels early in their fight against Assad. But Mr Obama resisted, arguing that there was too much uncertainty about the composition of the rebel forces. He also expressed concern about adding more firepower to an already bloody civil war.
Separately, the White House announced that it was providing 25 million US dollars (£15.5 million) in immediate military assistance to the Iraqi government as part of efforts to combat the Islamic State.
In the hours before the president's remarks, the US Treasury Department said that Mr Obama's strategy would include stepped-up efforts to undermine the IS group's finances. David Cohen, the treasury's undersecretary for terrorism and financial intelligence, wrote in a blog post that the US would be working with other countries, especially Gulf states, to cut off the group's external funding networks and its access to the global financial system.
The US has also been pressing allies in Europe, the Middle East and elsewhere to help with efforts to degrade the terror group.
France's foreign minister said that his country was ready to take part in air strikes against extremist fighters in Iraq if needed. And the German government announced that it was sending assault rifles, ammunition, anti-tank weapons and armoured vehicles to Kurdish forces in Iraq fighting, breaking with Berlin's previous reluctance to send weapons into conflicts.
US secretary of state John Kerry is travelling to Saudi Arabia and Jordan this week. He first made a stop in Baghdad to meet with Iraq's new leaders and pledge US support for eliminating the extremist group.