Top commander warns Syria will ‘break apart’ post-Assad

“When people say, ‘what does Syria look like the day after Assad?’ That's the wrong question. It's not what it will look like the day after; it's what will Syria look like a decade after,” Stavridis added.

Stavridis made the comments as the White House is weighing action against Syria’s leaders after reports of chemical weapons use and with the Pentagon and U.S. agencies already preparing transition plans for a post-Assad Syria.

Pentagon press secretary George Little on Monday discussed those efforts, saying "This is a combustible region and [the Defense Department] is aware of that.”

The mainly Sunni opposition forces in Syria are waging a three-year battle to oust Assad’s regime from power.

The battle has increasingly turned to a stalemate, as government troops loyal to Assad batter rebel forces with airpower, heavy artillery and possibly chemical weapons, according to U.S. intelligence reports.

But Assad has also carved out significant support among the country's ruling Alawite population, a strain of Shiite Islam the Assad family practices, in western Syria.

When Assad falls, Middle East watchers fear it could invite a prolonged civil war as Sunnis and Alawites square off to fill the power vacuum in the country, mirroring the bloodshed seen in post-Saddam Hussein Iraq.

The influx of terror groups from neighboring Iraq, as well as the increasing presence of fighters from the Iranian-backed militant group Hezbollah, has only complicated the situation for a post-Assad Syria.

Little on Monday called Syria “one of the most complex [national security] crises of the last generation.”

But reports the Assad regime has used chemical weapons, crossing a "red line" set by President Obama, has fueled congressional calls for U.S. action.

A bipartisan group of four senators on Thursday went to the Senate floor to pressure Obama to take more aggressive actions with Syria.

Sens. John McCainJohn Sidney McCainObama, Bush veterans dismiss Trump-Putin interpreter subpoena Controversial Trump judicial nominee withdraws Trump vows to hold second meeting with Putin MORE (R-Ariz.), Carl LevinCarl Milton LevinConservatives see Kethledge as 'Gorsuch 2.0' How House Republicans scrambled the Russia probe Congress dangerously wields its oversight power in Russia probe MORE (D-Mich.), Lindsey GrahamLindsey Olin GrahamQuestions mount over Trump-Putin discussions The Hill's Morning Report — Trump and Congress at odds over Russia Overnight Defense: Trump inviting Putin to DC | Senate to vote Monday on VA pick | Graham open to US-Russia military coordination in Syria MORE (R-S.C.) and Bob MenendezRobert (Bob) MenendezCNN anchors break into laughter over comedian's alleged prank call to Trump Comedian claims he tricked Trump while impersonating Dem senator Schumer: Obama 'very amenable' to helping Senate Dems in midterms MORE (D-N.J.) all said Obama should put military pressure on the Syrian regime by arming rebel groups or launching airstrikes against the country's defenses.

Levin, the chairman of the Senate Armed Services Committee, said it was “essential” that the U.S. “step up the military pressure on the Assad regime.”

But Stavridis openly questioned the benefits of U.S. military action in Syria, warning it could lead to a prolonged U.S. intervention in the country — one that could cost U.S. lives.

"One strike is a very different proposition than launching a big campaign," said the four-star admiral, who oversaw the American and NATO military intervention in Libya.

"The benefit of surprise and stealth and a single-point strike may or may not tell us a good deal about Syrian air defense," Stavridis cautioned.

The best thing Washington and its allies can do in Syria is to keep the war from bleeding over into Turkey, a NATO ally, he added.

"NATO has to protect the NATO border. We have to ensure that Turkey is secure and that this doesn't spill into the Turkey border," said Stavridis.

He added American war planners' hands were essentially tied "until a political decision is made" by the White House on what the next step should be.

Stavridis is scheduled to step down from his command post later this year.