Marine Gen. James Mattis, the outgoing commander of U.S. troops in the Middle East and South Asia, told a Senate panel on Tuesday that he's got "options prepared" to secure Syria's deadly chemical arsenal.

Mattis conceded that it would be "very difficult" to prevent a chemical attack by dictator Bashar Assad's forces. But Mattis told the Senate Armed Services Committee that after his forces detected an initial chemical attack, they might "disrupt" a poison-gas onslaught and "might be able to further affect their transfer or use."

That's a marked shift in attitude, at the very least, from previous U.S. military assessments. Those assessments, often from Pentagon officials, typically hold that securing Assad's chemical precursors or weapons would require a force of 75,000 troops – a size considered prohibitive, as it's larger than the one in Afghanistan. In January, Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff said it would be "almost unachievable" to prevent Assad from launching a chemical attack, owing to the likelihood of insufficient prior intelligence.

Mattis, who has previously sounded more hawkish on Syria than the Obama administration, did not publicly discuss what his "options" include. His former boss, ex-Defense Secretary Leon Panetta, stated in January that "we're not working on options that involve boots on the ground" to take custody of those chemical weapons. Panetta said that the U.S. would only send troops to Syria to secure its extensive chemical stockpiles pending an open invitation by a post-Assad government, "but a hostile situation, we're not planning for that."

Yet Mattis expressed doubt that there will be a stable post-Assad Syria. Iran would view its proxy Assad's downfall as "the biggest strategic setback for Iran in 25 years" and begin arming and training militias for a "Lebanese Hezbollah-type effect." Mattis said the Iranian Revolutionary Guard Corps is "both on the ground [in Syria] and are bringing in foreign fighters."

All that creates the prospect that Iran and its allies would seek to fight U.S. forces for control of Assad's chemical arsenal – and that Mattis' plans might have to contend with sending troops onto an active chemical battlefield.

Mattis won't be there to implement them, even if they're ultimately implemented. Last week, the Senate panel approved his successor at U.S. Central Command, Army Gen. Lloyd Austin, whose nomination is now before the full Senate. Tom Ricks at Foreign Policy reported that the White House ousted Mattis, in part due to disagreements on handling Iran, something the White House disputes. (Notably, Mattis answered "No, sir" when asked if the administration's strategy to forestall a nuclear Iran was working.)

Whatever Mattis' plans to mitigate a Syrian chemical attack, he told the panel they don't include the Syrian opposition. Mattis expressed concern that arming them risked putting weapons into the hands of "our enemies." Although, Mattis left the door open to other U.S. agencies working alongside the Syrian rebels, telling senators that any such collaboration was "not by U.S. Central Command."