A protester steps on a poster of Syrian President Bashar al-Assad during demonstrations in London: World leaders are concerned Assad will tap into the country's stockpile of chemical weapons if he feels cornered.

Bimal Gautam/xh/Xinhua Press/Corbis

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yrian rebels trying to overthrow the regime of President Bashar al-Assad said on Monday that they have shot down a government fighter jet for the first time, a claim disputed by Syria's military, which blamed technical failures for the plane's crash. The rebels posted videos to YouTube that they say bolsters their case. If true, the rebels' downing of a jet would be a big blow to the pro-Assad military. "Regardless of how they did it, if they can put down a jet fighter, then they can put down other planes as well," says Sami Nader, a professor of international relations at St. Joseph University in Beirut. "Assad's trump card was the military, but he is now losing this last card."