With that tension in mind, take a look at Obama's careful presentation to the U.N. of American response to the Arab Spring (my emphasis):
It has been less than two years since a vendor in Tunisia set himself on fire to protest the oppressive corruption in his country, and sparked what became known as the Arab Spring. And since then, the world has been captivated by the transformation that’s taken place, and the United States has supported the forces of change.We were inspired by the Tunisian protests that toppled a dictator, because we recognized our own beliefs in the aspiration of men and women who took to the streets.We insisted on change in Egypt, because our support for democracy ultimately put us on the side of the people.We supported a transition of leadership in Yemen, because the interests of the people were no longer being served by a corrupt status quo.We intervened in Libya alongside a broad coalition, and with the mandate of the United Nations Security Council, because we had the ability to stop the slaughter of innocents, and because we believed that the aspirations of the people were more powerful than a tyrant.And as we meet here, we again declare that the regime of Bashar al-Assad must come to an end so that the suffering of the Syrian people can stop and a new dawn can begin.
This precis includes is a submerged narrative chronicling U.S. caution and calculation even as "we recognized our own beliefs in the aspiration of men and women who took to the streets." The slow withdrawal of U.S. support for Mubarek is captured in one word: ultimately. In Yemen, our stance changed with the times; apparently, the people's interest was formerly served better by the status quo than by any visible alternative (interesting, too, that Tunisia's Ben Ali is dismissed as a dictator, while the name of Yemen's Saleh, long a "partner" in the U.S. war against al Qaeda in the Arabian Peninsula, is not attached to "the corrupt status quo").
So there is an implicit distinction between a past in which U.S. support for dictators may have been defensible to the moment(s) when the people provided a choice (at which point, Churchill might murmur, the U.S. did the right thing after exhausting the alternatives). The U.S. supports democracy, rule of the people, when the will of the people reaches critical mass. And there's a parallel distinction between what the U.S. "had the ability" to do in Libya and what it can only "declare" must happen in Syria -- with the subtext, perhaps, that Obama also "declared" that Gaddafi had to go before eventually making it happen, under U.N. auspices.