Syria and Iran are hardly unrelated problems. In the minds of many on President Obama’s team, nothing would undercut Iran’s capability to cause trouble in the region faster than if the mullahs lost Bashar al-Assad, Syria’s brutal president, as their only ally in the Arab world. The argument commonly heard inside and outside the White House these days is that if the Assad government cracks, Iran’s ability to funnel weapons to Hezbollah and Hamas will be badly damaged — and its influence will wither accordingly. Similarly, if Iran’s effort to walk up to the edge of a nuclear weapons capability can be set back with a few well-placed GBU-31 bunker-busters, the country’s hopes of challenging Israel and Saudi Arabia to be the region’s biggest power will be deferred.

Or so the theory goes.

Usually the appeal of providing arms and technology for someone else to do the fighting is undeniable. It’s why Franklin D. Roosevelt invented Lend-Lease to provide planes, tanks and ammunition to the British in 1941 when they were broke, and it’s how Ronald Reagan got into trouble in the Iran-contra deal, an effort to arm Nicaraguan rebels by diverting funds from a secret arms deal with the country Washington is now sanctioning and sabotaging. At a moment when polls show the country has had its fill of ground wars and the White House talks of “nation-building at home,” there is something tempting about handing off weaponry to the rebels and the Israelis, wishing them good luck and reminding them to drop a line back to the White House if any of it works.

“If it was only that easy,” one senior national security official told me last week.

The first question that White House officials say they are asking about the Syrian rebels is the same question they asked about Libya 10 months ago: Who are these guys?

In Libya, Mr. Obama took a pass on arming the rebel fighters, electing to join in a NATO air campaign instead. (As it turned out, the United Arab Emirates provided a large number of small arms to help overthrow Col. Muammar el-Qaddafi.)

In Syria, where the death toll is already above 6,000 by most estimates, there is no equivalent NATO operation; so far, a limited intervention to spur a coup or create a “safe zone” for Syrian civilians near the Turkish border is all still talk. So at first glance, providing arms looks like the next-best option. But the worry is that what started as a protest movement has morphed into what Steven Heydemann, a Syria expert at the United States Institute of Peace, described as “a dangerous and uncoordinated array of armed opposition fighters.” While there is an entity called the Free Syrian Army — not to be confused with the civilian Syrian National Council — it is less an army than bands of free-form militias. Some are tribal; some are linked by regional or ethnic bonds; there is no real command structure.