Free Syrian Army fighters stand guard at a checkpoint in Idlib province, Syria. (AP/Hussein Malla)

About two years into the Syrian conflict that has claimed tens of thousands of lives, President Obama "is preparing to send lethal weaponry to the Syrian opposition," The Washington Post's Karen DeYoung reported late on Tuesday. She cites administration officials as saying that Obama "has taken steps to assert more aggressive U.S. leadership among allies and partners seeking the ouster of President Bashar al-Assad."

Some, though clearly not all, senior U.S. officials have long favored a plan to provide arms to Syrian rebels. Those public proponents included, as of February, then-Defense Secretary Leon E. Panetta, then-Secretary of State Hillary Rodham Clinton, the chairman of the Joint Chiefs of Staff and former CIA director David H. Petraeus. And yet Obama, still skeptical, showed no sign of bending.

So what changed? Why would the Obama administration now consider a move it has so long resisted?

There are, at the moment, three prevailing theories for the apparently shifting White House strategy. The real answer may well be some combination of these, perhaps with a touch of internal politics or other less-strategically oriented factors.

1. An earnest response to Syrian chemical weapons reports

Obama administration officials have been clear that the use of chemical weapons in Syria is a "red line" for them, although they've been consistently vague on what happens if the line is crossed. Late last week, the administration revealed that U.S. intelligence had found signs that some Syrians had been exposed to sarin, although it's not clear whether it was due to a deliberate attack.

When Obama said at a news conference Tuesday that it would be a "game changer" if Syria had used chemical weapons, a reporter asked, "By game changer, do you mean U.S. military action?" The president responded, "By game changer, I mean that we would have to rethink the range of options that are available to us." At the time, it was easy to take this as a brush-off, but perhaps Obama really is rethinking whether the United States should directly arm the rebels, and he could be moving more in that direction as more evidence of chemical weapons use emerges.

2. A slow evolution

The deaths have been mounting for months now, and the fighting intensifying, with no clear sign that the rebels are headed for sure victory. Meanwhile, Islamists, some of them allied with al-Qaeda, are increasingly prominent among the rebels. Perhaps these trends have gradually led Obama to become more sympathetic to the idea of directly supplying  arms, if only to hasten the conflict toward its conclusion.

In this theory, the chemical weapons reports may have edged Obama toward wanting to push the conflict closer to its end, or it may simply have provided a politically opportune moment for revealing the potential policy shift.

3. A warning to Russia

It sounds like a conspiracy theory, but such writers as Barbara Slavin of the Atlantic Council and Foreign Policy's Blake Hounshell have wondered if the announcement might be timed to influence Russia's calculus on whether it should continue to support Syrian leader Bashar al-Assad. Moscow has long backed Assad, who needs its support to remain armed and in power. Obama, by dropping hints that he is considering arming the Syrian rebels, might tip Russia in the direction of abandoning Assad. If Russian President Vladimir Putin can be convinced that Assad's fight is hopeless and that the status quo could lead to American involvement -- something Putin would surely rather avoid -- then perhaps he might back away from Assad.

Administration officials seemed to suggest, in talking to The Post's DeYoung, that arming the Syrian rebels could be partially contingent on failed diplomacy with Russia, thus putting a bit more of the burden on Moscow. Here's the second paragraph of her story:

The officials said they are moving toward the shipment of arms but emphasized that they are still pursuing political negotiation. To that end, the administration has launched an effort to convince Russian President Vladimir Putin that the probable use of chemical weapons by the Syrian government — and the more direct outside intervention that could provoke — should lead him to reconsider his support of Assad.

Or, alternatively, it could all be a head fake. Obama has reportedly not yet made a decision, but will within "weeks." Check back for more on this.