President Obama outlined a four-step plan on combating the threat of the Islamic State. Here are the highlights from his speech. (Nicki DeMarco and Ashleigh Joplin/The Washington Post)

President Obama on Wednesday night outlined an open-ended campaign to combat the threat posed by the Islamic State, significantly expanding the counter­terrorism strategy that has been a hallmark of his presidency.

Obama said in a prime-time speech televised from the White House that the United States will join “with our friends and allies to degrade, and ultimately destroy, the terrorist group known as ISIL,” using an alternative acronym for the group that has emerged in Iraq and Syria.

Saying the United States is meeting the threat with “strength and resolve,” the president also sought to assuage the concerns of Americans who are wary of another foreign entanglement, insisting that the offensive against the militant group will not involve combat troops but will rather be a “steady, relentless effort” conducted through airstrikes in both Syria and Iraq, and by supporting military partners on the ground.

“I want the American people to understand how this effort will be different from the wars in Iraq and Afghanistan. It will not involve American combat troops fighting on foreign soil,” he said. Obama compared the new initiative to smaller-scale fights the United States has engaged in. “This strategy of taking out terrorists who threaten us, while supporting partners on the front lines, is one that we have successfully pursued in Yemen and Somalia for years,” he said.

But the scope of the new operation — which will immediately involve expanded airstrikes, additional U.S. personnel in Iraq and new support for moderate Syrian rebels — is likely to overshadow those two efforts. In the 13-minute address, Obama did not give a fixed date for when the operation might end, and his top aides have suggested it might last beyond his time in office.

The president says he has the legal authority to conduct the expanded military operations without new congressional approval. “This is a core principle of my presidency: If you threaten America, you will find no safe haven,” he said.

Still, he has asked Congress to explicitly authorize U.S. military personnel to train Syrians, Iraqis and others to combat the Islamist militants in both countries, saying, “I welcome congressional support for this effort in order to show the world that Americans are united in confronting this danger.”

Saudi Arabia has agreed to host and help fund the training program, according to White House officials. Obama called Saudi King Abdullah on Wednesday to discuss the operation.

Senate Majority Leader Harry M. Reid (D-Nev.) signaled on Wednesday that he supports Obama’s approach. “It’s clear to me that we need to train and equip Syrian rebels and other groups in the Middle East that need some help,” Reid said.

Obama had initially resisted arming the rebels but asked in June for lawmakers to give the Pentagon — as opposed to the CIA — the authority to train and equip them. He also asked for $500 million to fund the effort.

House Speaker John A. Boehner (R-Ohio) issued a statement Wednesday night saying Obama “recanted his earlier dismissals of ISIL’s capabilities and rightly acknowledged the grave and growing threat posed by the spreading global epidemic of radicalized Islam.”

Boehner said he remained concerned that even as Obama made “a compelling case for action,” the training program “could take years to fully implement” and the strategy may not go far enough.

The War Powers Act of 1973 is a classic separation of powers struggle. President Obama has already taken military action to fight the Islamic State in Iraq, but is Syria next? Here’s what the president can do, with or without Congress. (Jackie Kucinich/The Washington Post)

On the left, some Democrats insisted Congress should vote on the expansion of the military operation. “I will not give this president — or any other president — a blank check to begin another land war in Iraq,” Sen. Mark Udall (D-Colo.) said in a statement.

Seeking to address some of the legal concerns, administration officials said the president could expand the airstrike campaign into Syria under the 2001 Authorization for Use of Military Force: That authorization allows for “all necessary and appropriate force against those nations, organizations, or persons” responsible for the Sept. 11, 2001, attacks and those who “harbored such organizations or persons.”

While the Islamic State is now at odds with al-Qaeda, the target of the original law, officials said the Islamic State’s “long-standing relationship” with al-Qaeda and Osama bin Laden is sufficient to be covered by the statute.

The administration’s new military strategy marks a shift to offense against radical Sunni militants in Iraq, as opposed to the defensive and humanitarian effort that has been underway since last month. The United States has conducted 154 airstrikes and has 1,043 troops stationed there, according to the Pentagon; Obama said he would send an additional 475 to Iraq to expand the mission.

A “broad coalition” would join the United States in fighting the Islamic State, Obama said. On Wednesday, French Foreign Minister Laurent Fabius said his nation would help in conducting airstrikes in Iraq and called for other countries to join in the international fight against “this transnational danger that could reach all the way to our soil.”

The State Department released a report Wednesday on the efforts that three dozen countries, from Albania to the United Arab Emirates, have undertaken to combat the Islamist militants or ease the humanitarian crisis in Iraq and Syria.

While Obama has come under fire from lawmakers in both parties after admitting that he had not yet formulated a strategy to deal with the Islamic State, his aides said the president was determined to take a deliberate approach and was waiting for key developments, such as the formation of a new government in Iraq. Secretary of State John F. Kerry met with Iraq’s new prime minister, Hai­der al-Abadi, on Wednesday.

Abadi, who called the Islamic State “a terrorist nation,” pledged “to include all people in the Iraqi society from all sections of this society in this government and in the mobilization process” as the country sought to establish regional stability.

Back at home, the president and his top aides have worked hard to muster bipartisan support for his plan. Obama called House Appropriations Committee Chairman Harold Rogers (R-Ky.) on Tuesday night to ask him to give him the legal authority to train foreign fighters, while some of the president’s senior national security advisers briefed a bevy of senior House members on the respective committees of jurisdiction Wednesday.

Rogers said it was the first time he had received such a call. “I don’t recollect a previous time,” he said. “In good faith, we’re trying to get briefed up on what the request is, and it’s a complicated, big-time change in policy that I’d hate to see us attach to a continuing resolution at the very last minute.”

While GOP leaders expressed frustration at the last-minute ­request, lawmakers are eager to embrace a strategy that will reverse the gains the Islamic State has made recently. Obama’s proposal would insert the measure into a broader bill funding federal agencies, which appeared poised for passage next week.

Announcing the spending bill’s delay on the House floor, Majority Leader Kevin McCarthy (R-Calif.) said, “We stand ready to listen and work with the president to confront this growing threat,” drawing thunderous applause from members of both parties.

Rep. Tom Cole (R-Okla.), a senior member of the House Appropriations Committee, predicted Wednesday that most House Republicans would support legislation to grant the president the authority to fund training for foreigners to fight the Islamic State.

“My impression is that he would get a strong bipartisan vote for just about anything he wants to do, and he should take advantage of that opportunity,” Cole said.

Some lawmakers from both parties, however, expressed concern that Obama missed a critical opportunity to bolster the Syrian opposition. Sen. Bob Corker (Tenn.), who as the top Republican on the Foreign Relations Committee helped write language in May 2013 that would have expanded aid to the moderate rebels in their fight against Syrian President Bashar al-
Assad, said this week’s request from the administration represented a “speck of sand” in terms of the situation there.

“You just worry — we’ve let them become so diminished,” Corker said.

And Rep. Gerald E. Connolly (D-Va.), who praised Obama in a telephone interview after the speech for outlining “a very bold and decisive strategy,” said the president’s rebel-training request is “the toughest task and, I also think, the toughest of the goals to achieve” because it is so difficult to determine which rebels are trustworthy.

While much of the public debate has focused on the military component of the White House’s new plan, Jon B. Alterman, who directs the Middle East program at the Center for Strategic and International Studies, said how the administration works with the newly formed Iraqi government as well as leaders in Saudi Arabia, Turkey and Europe — and in parallel with Iran — will be critical.

“The mistake is thinking this is principally a military operation,” he said. “This is principally a political operation about what political entity controls a broad swath of territory.”

And even as the president emphasized the United States had a compelling interest in dismantling the Islamic State, his top deputies said there is no credible information that the Islamic State is planning to attack the United States.

Homeland Security Secretary Jeh Johnson said in a speech Wednesday that the group poses “a certain level of danger that constitutes a threat” to vital U.S. interests and that his agency, the FBI and other intelligence agencies are “making enhanced and concerted efforts” to track Syrian foreign fighters who leave or wish to enter the United States.

Paul Kane, Katie Zezima and Robert Costa contributed to this report.