At the same time, the two made clear that strikes weren't imminent. The Pentagon had begun war-gaming various scenarios, Dempsey said, but had yet to present them to the president. Panetta said that the U.S. was still working to assemble an international coalition against Assad so that Washington wouldn't have to act alone.

Dempsey also pointed out that Syria's air defenses were five times more sophisticated as those in Libya, making airstrikes riskier and more complicated. Panetta, for his part, said that the systems had been set up in heavily populated areas, which meant that American strikes could cause "severe collateral damage."

The testimony came as the Syrian crisis - and the international debate about how to handle it - continued to escalate. Outside groups estimate that Assad's forces have killed at least 7,500 people and effectively leveled rebel-held cities such as Homs. The U.S. and its allies have slapped hard-hitting economic and political sanctions on Damascus, but Russia and China have prevented the United Nations Security Council for authorizing stronger measures.

Here at home, an array of prominent senators is calling for the U.S. to do more to stop the bloodshed. Sens. John McCain, R-Ariz., Joe Lieberman, ID-Conn., and Lindsey Graham, R-S.C., have pressed for U.S. military involvement in Syria, with McCain this week explicitly calling for American airstrikes against Syrian forces.

At Wednesday's hearing, McCain criticized the administration for not acting more quickly and aggressively to force Assad out of power.

"In past situations, America has led. We're not leading, Mr. Secretary," McCain told Panetta early in the hearing. Later, McCain asked, "How many more have to die? 10,000 more? 20,000 more?"

Panetta and Dempsey made it clear that they shared McCain's outrage at Assad's continued slaughter and that they believe that he long ago lost any legitimate claim to power. Panetta likened the brutality and scope of Assad's crackdown to the violence unleashed by Chinese forces when they crushed pro-democracy protests in Beijing's Tiananmen Square in the 1990s.

But the secretary and the chairman stressed again and again that using the American military to push Assad from power would be far more challenging than similar humanitarian interventions into Libya and Bosnia. In addition to the size and sophistication of Syria's air defenses, they pointed out that Syria has a large military; the active assistance of Iran, which is shipping antitank missiles and other armaments into the country; and a stockpile of chemical and biological weapons a hundred times larger than that in Libya.

They also warned that the U.S. was unsure of the exact makeup of the Syrian rebel groups, including whether they had ties to al-Qaida or other extremist groups. Panetta and Dempsey said that the rebels didn't appear to have the kind of clear hierarchy and well-organized leadership structure that existed in Libya.

Still, the Syria conundrum won't end anytime soon. Assad's forces have crushed the opposition inside Homs and have clear momentum on the ground. The Obama administration faces a difficult and unwanted choice: Intervene militarily despite the clear risks of doing so, or rely on sanctions and diplomatic pressure despite the clear risks of failing to stop an unfolding humanitarian disaster.



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