Marine Gen. James Mattis, the commander of U.S. forces in the Middle East, didn't come out and endorse U.S. intervention in Syria. But he did everything short of it, playing up the opportunity for the U.S. to bloody the nose of an Iranian proxy. Mattis told a Senate panel that knocking off dictator Bashar Assad would represent "the biggest strategic setback for Iran in 25 years."

A proxy war against Iran sounded like the most compelling rationale for involvement in Syria to Mattis, who testified on Tuesday morning before a Senate Armed Services Committee strongly concerned with aiding the Syrian rebels and checking Iranian influence in the Mideast. Prompted primarily by Sen. John McCain, who endorses U.S. airstrikes on Assad's forces, Mattis said Iran's "link to Lebanese Hezbollah will be cut if Bashar Assad goes," and that "providing arms" to the Syrian opposition "is perhaps an option."

It sounded like Mattis has considered the prospect of intervention in Syria with some specificity. Mattis said Syria lacked physical terrain conducive to providing "safe zones" for civilians under fire from Assad loyalists, as the former State Department policy chief Anne-Marie Slaughter has proposed. He said it wouldn't be easy: Russia has given Iran "very advanced" aerial defense systems, making "the imposition of any no-fly zone challenging." Asked if the White House has asked him to propose plans for Syria, Mattis said he didn't want to answer publicly.

"If we were to provide options, whatever they are, to hasten the fall of Assad," Mattis testified, then as long as other nations were on board, "it would cause a great deal of concern and discontent in Tehran."

And that was central to Mattis' presentation of any value to U.S. interests for intervening in Syria. (Not, say, securing chemical and biological weapons.) The real strategic goal would be to weaken Iran, which Mattis said "presents the most significant threat in the region." (Read: a greater threat than al-Qaida.)

Worldwide, Iran fights "basically a shadow war everyday," Mattis said, knocking off a list of nations in which Iran intervenes, from Sudan to Yemen to Iran to the crazy bomb plot to kill a Saudi ambassador in Washington. "They've gone all over the place, sir, and they enjoy this sort of thing," he told Sen. John Cornyn.

Mattis didn't endorse a bombing campaign – by either the U.S. or Israel – on Iran. And hawkish Israelis might not appreciate Mattis' comment that "only the Iranian people" can end Iran's nuclear problem. But he suggested that a confrontation with the Iranians would occur sooner or later, as the regime's behavior carries a "high potential for miscalculation" and it was something the U.S. has to "accept as part of their modus operandi."

All of this has been further than the White House has gone, either on Syria or on Iran. While the U.S. intelligence community doesn't think Iran's nuclear program is geared toward a bomb, Mattis said the Iranians were "enriching more uranium than they need for any peaceful purpose." And the White House thinks using U.S. military force in Syria could "accelerate the conflict on the ground and worsen the humanitarian situation without stopping the violence," as an official told ABC's Jake Tapper on Monday. However, Josh Rogin reports for Foreign Policy that the administration will provide "direct humanitarian and communications assistance to the Syrian opposition," while still ruling out military measures.

On Wednesday, the same panel will call Defense Secretary Leon Panetta and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, to testify on Syria. They may not be as comfortable with the prospect of getting involved in a whole new Mideast conflict. "The focus at this point remains on applying very strong diplomatic and economic pressure on the Syrian regime to stop what it's doing," says George Little, the Pentagon's chief spokesman.