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Secretary of State John Kerry made a forceful case for military action in Syria during congressional testimony on Tuesday, arguing that doing nothing would open a "Pandora's box" of "dangerous consequences" for the United States.

His testimony, along with that of Defense Secretary Chuck Hagel and Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey, came as the administration earned high-level backing on Capitol Hill for its call to use military force. House Speaker John Boehner and other congressional leaders endorsed a military strike to punish the Assad regime for a chemical weapons attack on Aug. 21.

But despite the apparent momentum, Kerry and the rest of the panel continued to face skeptical lawmakers who worried whether a "limited" strike would have limited impact, and whether it could nevertheless draw the U.S. deeper into the conflict.

"There are troubling questions that need to be answered," Sen. Tom Udall, D-N.M., said.

In a related development, a draft Senate resolution authorizing Obama to use military force against Syria, made public Tuesday night, would bar American ground troops for combat operations and set a deadline for any action.

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The measure, which the Foreign Relations Committee will vote on Wednesday, would set a time limit of 60 days and says the president could extend that for 30 days more unless Congress has a vote of disapproval.

Kerry spent much of the hearing walking back a remark that seemed to leave the door open to American ground troops going into Syria.

Kerry initially stirred confusion by saying he'd prefer not to include a prohibition on American troops going into Syria. He said that "in the event Syria imploded" or there was a threat of chemical weapons falling into the wrong hands, the U.S. and its allies would want to prevent that. "I don't want to take off the table an option that might or might not be available to the president of the United States to secure our country," he said.

Kerry later clarified that there "will not be American boots on the ground with respect to the civil war."

He again clarified, toward the end of the hearing, that he doesn't want anyone to "misinterpret" his remarks, and that the resolution considered by Congress "should not have any allowance for any troops on the ground."

Kerry got into a heated exchange with Sen. Rand Paul, R-Ky., who told Kerry that he hasn't talked to anybody who supports the call for military action. "Nobody's calling [my office] in favor of this war," he said.

"We don't believe we are going to war in the classic sense," Kerry said. "The president is not asking you to go to war."

Kerry argued that inaction in this case, though, would leave U.S. allies questioning the worth of America's word and U.S. enemies itching to test America's resolve. Kerry claimed the evidence is clear the Assad regime used chemical weapons and said America cannot "be spectators to slaughter."

"This is not the time for armchair isolationism," Kerry told the Senate Foreign Relations Committee.

Though tens of thousands have been killed in the course of the Syrian civil war, Obama administration officials and top-ranking lawmakers are arguing that it is now in the "national security interest" for the U.S. to get more involved, and punish the Assad regime for using chemical weapons.

Kerry as well as Senate Foreign Relations Committee Chairman Bob Menendez, D-N.J., said that failing to do so would embolden America's enemies. Menendez said failing to act would send Iran and others the message that the U.S. is "not serious" about its own goals of stopping the proliferation of chemical weapons.

"At the end of the day, our national security is at stake," Menendez said.

Kerry said the evidence shows "beyond a reasonable doubt" that the Assad regime used chemical weapons. He said failing to act would increase the risk that other regimes will do the same.

"Iran is hoping you'll look the other way," he said.

Both Kerry and Hagel tried to assure lawmakers that any military strike would be "limited" and not involve boots on the ground.

The hearing comes after the Obama administration won support for a strike on Syria from top congressional leaders. Boehner, emerging from a White House meeting, said the chemical weapons attack last month "has to be responded to."

"The use of chemical weapons is a barbarous act. It's pretty clear to me that the United Nations is unable to take action, NATO not likely to take action. The United States for our entire history has stood up for democracy and freedom for people around the world," Boehner said. "I'm going to support the president's call for action."

Minutes later, House Democratic Leader Nancy Pelosi told reporters that she, too, feels Syrian President Bashar Assad has "crossed a line."

House Republican Leader Eric Cantor said definitively that he plans to vote in favor of giving Obama authorization to use military force.

Senate Democratic Leader Harry Reid has also indicated his support.

The only hold-out appears to be Senate Republican Leader Mitch McConnell.

He said in a written statement that "while we are learning more about his plans, Congress and our constituents would all benefit from knowing more about what it is he thinks needs to be done-and can be accomplished-in Syria and the region."

The statements backed Obama's argument that a limited missile strike on the Assad regime would be in the U.S. national security interest, by deterring the use of chemical weapons. Obama, as the White House meeting began, said he was confident he would win congressional support.

Others, though, have cast the president's decision to seek congressional backing as a big risk. Some have described his chances of winning approval as "50-50." While congressional leaders and senior members of key committees are getting onboard, the president faces skepticism from both liberal Democrats and some Tea Party-aligned Republicans. Others worry the administration is not going far enough with a limited strike that the administration admits is not aimed at regime change.

Sen. John Cornyn, R-Texas, on Tuesday urged Obama to deliver a national address explaining to the American people why the U.S. should launch an attack.

"The president needs to explain in detail what vital national interests are at stake, his plan for securing these interests and a clear definition of what success looks like in Syria," he said in a written statement.