President Barack Obama used a nationally televised address Tuesday night to make his case for military action against Syria, telling skeptical Americans that President Bashar Assad's government posed a threat to U.S. security even as he recognized that diplomatic steps could render attacks unnecessary.While noting he had asked Congress to postpone action on Syria while diplomacy unfolded, Obama also said he has ordered the U.S. military to remain prepared to carry out attacks if needed. He blamed last month's chemical attacks near Damascus squarely on Assad and warned that a failure to act now would encourage tyrants and terrorists to use similar weapons."Our ideals and principals 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 said.Obama's speech was seen as a critical one for his presidency, though for reasons different than when plans for it were announced last week. It was intended as the climax of the administration's pitch to persuade Congress to endorse military action in Syria. Polls showed Americans, wary of another Middle East conflict, oppose military action and Obama, lacking support from liberals and conservatives alike, was in danger of losing the congressional vote.Syria's announcement that it would accept a Russian plan to turn over the chemical weapons stockpile posed a new challenge to Obama: whether to press arguments for military strikes or ease up because of the prospects of a diplomatic breakthrough. Earlier in the day, Obama hurriedly dispatched his top diplomat, Secretary of State John Kerry, to Geneva for talks Thursday with Russian Foreign Minister Sergey Lavrov.But the speech clearly put the emphasis on the military action, not diplomacy. Most of the remarks - which lasted only about 15 minutes - were making the case for the U.S. to respond."America is not the world's policeman," Obama said. "Terrible things happen across the globe, and it is beyond our means to right every wrong. But 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."The unpredictable chain of events stemmed from a chemical weapons attack in the Damascus suburbs on Aug. 21. U.S. officials say more than 1,400 people died, 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 U.N. Security Council behind a military strike. But Monday, after a remark by Kerry, Russian officials 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 is prepared to implement a Russian proposal to put its chemical weapons arsenal under international control.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, and he has consistently declined to say whether he would do so if lawmakers refuse to approve the legislation he is seeking.The lukewarm support in Congress was underscored on Tuesday when liberal Democratic Sen. Ed Markey and conservative Republican Rep. Mark Mulvaney both announced their opposition.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, House Speaker John Boehner and Majority Leader Eric Cantor, the top two Republicans in the House of Representatives, and House Democratic leader Nancy Pelosi have endorsed Obama's request.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."