Obama's decision to launch attacks inside Syria, which is embroiled in a three-year civil war, marked a turnabout for the president, who shied away a year ago from airstrikes to punish Syrian President Bashar al-Assad for using chemical weapons against his own people.
In a widely anticipated, 13-minute White House speech, Obama said he would hunt down Islamic State militants "wherever they are" in a drive to degrade and ultimately destroy the group, which has seized broad stretches of Iraq and Syria.
"That means I will not hesitate to take action against ISIL in Syria, as well as Iraq. This is a core principle of my presidency: if you threaten America, you will find no safe haven," he said, speaking on the eve of the 13th anniversary of the Sept. 11, 2001, attacks.
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Obama asked Congress to authorize $500 million to train and arm "moderate" Syrian rebels. The training would take place in Saudi Arabia.
It is unclear whether more American weapons and training can shift the battlefield balance toward the US-backed rebels, who are badly outgunned by Islamic State, other militant groups and Assad's forces.
A vote on the money would put lawmakers on record supporting the military action, although White House officials stressed Obama already had the authority he needed for the new moves.
Obama plans to expand the list of targets inside Iraq beyond several isolated areas. The US military has launched more than 150 airstrikes in Iraq in the past month to help halt Islamic State advances.
The new target list will include Islamic State's "leadership, logistical and operational capability," as well as an attempt to "deny it sanctuary and resources to plan, prepare and execute attacks," the White House said.
US officials have warned it will take years to destroy Islamic State, and Obama told Americans: "It will take time to eradicate a cancer like ISIL," the White House's acronym for the militant group.
More US military advisorsObama will send 475 more American advisers to help Iraqi forces, which will bring to 1,600 the number there. Obama, determined to avoid a repeat of the Iraq war, stressed they would not engage in combat.
The president laid out his emerging plan for tackling the group two weeks after coming under fire for saying: "We don't have a strategy yet" for the group in Syria and six months after declaring that groups like Islamic State were minor players.
The US view of the threat from Islamic State now is that foreign fighters who have sworn allegiance to the group could return to their home countries and launch attacks against civilian targets, including in the United States. Islamic State fighters beheaded two American captives in the past month, shocking Americans who have demanded Obama retaliate.
"Our intelligence community believes that thousands of foreigners - including Europeans and some Americans - have joined them in Syria and Iraq. Trained and battle-hardened, these fighters could try to return to their home countries and carry out deadly attacks," Obama said.
Republican lawmakers welcomed what they said was a tardy recognition that Islamic State represented a threat to the United States.
"A speech is not the same thing as a strategy, however," said House of Representatives Speaker John Boehner, the top U.S. Republican. "While the president presented a compelling case for action, many questions remain about the way in which the president intends to act."
Training camps in Saudi ArabiaIn a significant move that could help rally Gulf Arab states behind the US-led coalition, key ally Saudi Arabia will host inside its territory a US training effort for Syrian rebels, senior US officials said.
The effort is dependent on the US Congress approving the $500 million for the rebels.
Congress is to consider the measure next week before lawmakers adjourn to campaign for Nov. 4 elections. The funding request generally has broad support.
The Saudi decision emerged after Obama spoke by phone earlier in the day with Saudi King Abdullah, who has pressed the American government to do more resolve the Syrian conflict.
Obama, vowing he would not send US combat forces back to the region, said he was building a broad anti-Islamic State coalition involving Sunni-led governments in the region and Western allies.
US officials want allies to join in attacks on the group as well as in training and equipping Iraqi forces and Syrian rebels, and providing humanitarian relief and intelligence.
Building an allied coalitionWhat specifically each nation will do in the coalition remains to be hammered out. Secretary of State John Kerry is meeting Gulf allies in the region and Obama is to host a leaders' security conference at the UN General Assembly in two weeks with the aim of fleshing out duties of the coalition.
Before the focus on Islamic State, Obama for months had been cool to the notion of arming the poorly organized Syrian rebels, fearing weapons provided them could end up in the wrong hands.
But he now needs the rebels to become strong enough to hold ground cleared by US airstrikes, just as Iraqi forces are doing in Iraq.
US officials pushed back hard against the notion that striking Islamic State strongholds in Syria would unintentionally help Assad. They said the Sunni-majority areas in the eastern part of the country the militants hold were not places where Assad loyalists would be able to take advantage to regain control.
Obama's speech included a dollop of election-year politics as he seeks to rally Democratic voters to prevent Republicans from seizing control of the US Senate. He said his policies had helped bring back the US economy from a severe economic crisis that greeted him when he took office in 2009.
"America is better positioned today to seize the future than any other nation on Earth," he said.