WASHINGTON — President Barack Obama told a war-weary nation Tuesday night that diplomacy suddenly holds “the potential to remove the threat of chemical weapons” in Syria without the use of force. But he also vowed the U.S. military will be ready to strike against President Bashar Assad if other measures fail.

For now, Obama said he had asked congressional leaders to postpone a vote on legislation he has been seeking to authorize the use of military force against Syria.

In a 16-minute speech, the president repeatedly offered reassurances that even the failure of diplomacy — in promised talks at the United Nations or elsewhere — would not plunge America into another war.

“I will not put American boots on the ground in Syria,” he promised. “I will not pursue an open-ended action like Iraq or Afghanistan. I will not pursue a prolonged air campaign like Libya or Kosovo.”

“This would be a targeted strike to achieve a clear objective: deterring the use of chemical weapons and degrading Assad’s capabilities,” he said.

The speech capped a frenzied 10-day stretch that began when he unexpectedly announced he was stepping back from a threatened military strike and instead asking Congress first to pass legislation authorizing the use of such force against Assad.

With public opinion polls consistently showing widespread opposition to American military intervention, the White House has struggled mightily to generate support among liberal Democrats and conservative Republicans alike who have expressed fears of involvement in yet another war in the Middle East and have questioned whether U.S. national security interests were at stake in Syria. Obama had trouble, as well, building international support for a military attack designed to degrade Assad’s military.

Suddenly, though, events took another unexpected turn this week. First Russia and then Syria reacted positively to a remark from Secretary of State John Kerry indicating that the crisis could be defused if Damascus agreed to put its chemical weapons under international control.

The president said he was sending Kerry to meet with Russian Foreign Minister Sergey Lavrov on Thursday, and he added, “I will continue my own discussion” with Russian President Vladimir Putin, with whom he has said he had been discussing ways out of the Syrian predicament for some time.

At the same time, he said the United States and its allies would work with Russia and China to present a resolution to the United Nations Security Council “requiring Assad to give up his chemical weapons and to ultimately destroy them under international control.”

“It’s too early to tell whether this offer will succeed, and any agreement must verify that the Assad regime keeps its commitments,” he said.

Acknowledging the weariness the nation feels after a decade of war in Iraq and Afghanistan, Obama said, “America is not the world’s policeman.”

And yet, he added, “When with modest effort and risk we can stop children from being gassed to death and thereby make our own children safer over the long run, I believe we should act. That’s what makes America different. That’s what makes us exceptional.”

“Our ideals and principles, as well as our national security, are at stake in Syria, along with our leadership of a world where we seek to ensure that the worst weapons will never be used,” he declared.

Obama recounted the events of the deadly chemical weapons attack on Aug. 21 that the United States blames on Assad.

“When dictators commit atrocities, they depend upon the world to look the other way until these horrifying pictures fade from memory. But these things happened. The facts cannot be denied,” he said.

The president said firmly that Assad’s alleged attack was “not only a violation of international law, it’s also a danger to our security.”

If diplomacy now fails and the United States fails to act, he said, “the Assad regime will see no reason to stop using chemical weapons” and “other tyrants will have no reason to think twice about acquiring poison gas and using” it. Over time, he added, U.S. troops could face the threat of chemical warfare, and if fighting escapes Syria’s border, “these weapons could threaten allies like Turkey, Jordan and Israel.”

In the run-up to the president’s speech, Defense Secretary Chuck Hagel pointedly told a congressional hearing it was not time to let the threat of military retaliation lapse. “For this diplomatic option to have a chance at succeeding, the threat of a U.S. military action, the credible, real threat of U.S. military action, must continue,” he declared.

At the same hearing, Kerry said any diplomacy “cannot be a process of delay. This cannot be a process of avoidance.”

He later added that any agreement must include binding consequences if Syria fails to comply, and lawmakers moved to rewrite pending legislation along the same lines.

The president readied his speech as a small crowd of anti-war protesters, some waving signs, gathered outside the gates of the White House.

U.S. officials say more than 1,400 died in the Aug. 21 episode, including at least 400 children, and other victims suffered uncontrollable twitching, foaming at the mouth and other symptoms typical of exposure to chemical weapons banned by international treaty. Other casualty estimates are lower, and Assad has said the attack was launched by rebels who have been fighting to drive him from power in a civil war that has so far claimed the lives of more than 100,000 civilians.

Assad’s patron, Russia, has blocked U.S. attempts to rally the Security Council behind a military strike. But Monday, after a remark by Kerry, it spoke favorably about requiring Syria to surrender control of its chemical weapons, and the Syrian foreign minister did likewise.

The foreign minister, Walid al-Moallem, said Tuesday that his government was ready to turn over its chemical weapons stockpile in line with Russia’s proposal in order “to thwart U.S. aggression.” He also said Syria was prepared to sign an international chemical convention it has long rejected — a step it can take on its own at any time without U.S. or U.N. supervision.

Syria has long refused to provide an accounting of the size of its stockpile, rarely referring in public to its existence. According to an unclassified estimate by the French government, it includes more than 1,000 tons of “chemical agents and precursor chemicals,” including sulfur mustard, VX and sarin gas.

Obama has said frequently he has the authority as commander in chief to order a military strike against Assad regardless of any vote in Congress.

The response in Congress to support such a strike has been lukewarm at best — as underscored during the day when liberal Sen. Ed Markey, D-Mass., and conservative Rep. Mick Mulvaney, R-S.C., both announced their opposition.

Markey, who was elected to the seat that Kerry vacated when he joined the Cabinet, said the legislation under consideration was too broad, “the effects of a strike are too unpredictable, and … I believe we must give diplomatic measures that could avoid military action a chance to work.”

And Rep. James Langevin, a Rhode Island Democrat who sits on committees dealing both with military and intelligence matters, said he feared that “Iran and Russia could cause serious damage” to the United States if they retaliated with a cyberattack.

Yet Maryland Rep. Steny Hoyer, the second-ranking Democrat in the House, said, “It would be inimical to our country’s standing if we do not show a willingness to act in the face of the use of chemical weapons and to act in a limited way to address that use alone.”

Earlier, Senate Republican leader Mitch McConnell became the first congressional leader to come out against legislation giving the president authority for limited strikes. “There are just too many unanswered questions about our long-term strategy in Syria,” he said.

By contrast, Speaker John Boehner of Ohio and Majority Leader Eric Cantor of Virginia, the top two Republicans in the House, have endorsed Obama’s request.

Given the uncertainty of diplomatic maneuvering, no vote is expected for several days, if then.

“If something can be done diplomatically, I’m totally satisfied with that. I’m not a blood and thunder guy. I’m not for shock and awe,” said Senate Majority Leader Harry Reid in a reference to the massive display of firepower that opened the war in Iraq nearly a decade ago.

Still, there was ample skepticism in Congress about the United Nations as well as Russia’s true intentions, as well as Syria’s willingness to be bound by international agreements.

“There is an overwhelming view it would be preferable if international law and the family of nations could strip Syria of the chemical weapons,” said Sen. Chuck Schumer, D-N.Y. “And there’s a large view we should let that process play out for a little while.”

Said Boehner: “Clearly, diplomacy is always a better outcome than military action. But I will say that I’m somewhat skeptical of those that are involved in the diplomatic discussions today.”

Associated Press writers Donna Cassata, Alan Fram, Stephen Ohlemacher, Lolita Baldor, Bradley Klapper, Nancy Benac, Deb Riechmann, Matthew Lee, Kimberly Dozier and Darlene Superville contributed to this story.