ONE OF THE foremost concerns expressed by members of Congress considering President Obama’s request for authorization of a military strike in Syria is that U.S. action will end up empowering actors who are even worse than Bashar al-Assad: in particular, al-Qaeda affiliates who have been prominent in fighting the regime. The worries are legitimate. But if they prompt legislators to oppose American military action, the threat to both Syria and U.S. national interests from the jihadists will grow worse.

As The Post and other news organizations have reported, al-Qaeda groups such as the al-Nusra Front and the Islamic State of Iraq and the Levant have used suicide bombings, beheadings and other brutal tactics to gain control over at least one provincial capital in northeastern Syria, as well as areas adjacent to the Turkish border. They have been prominent in battles between regime and opposition forces in areas across the country. They are determined to create a safe haven for al-Qaeda in Syria — like that which existed in Afghanistan before Sept. 11, 2001 — while imposing a Taliban-style fundamentalist regime.

However, the strength of the al-Qaeda forces has been exaggerated. Foremost among the myth-mongers is Russian President Vladi­mir Putin, a staunch Assad supporter who on Wednesday claimed that “al-Qaeda units are the main military echelon” of the Syrian rebels and that Secretary of State John F. Kerry lied to Congress when he asserted otherwise.

In fact, if anyone is willfully distorting the truth it is Mr. Putin. Both U.S. and independent intelligence estimates show that the 11 jihadist groups identified in Syria make up a small minority of the anti-government forces. U.S. officials say they constitute 15 percent to 25 percent of fighters. Elizabeth O’Bagy, an analyst at the Institute for the Study of War who has travelled extensively inside Syria, reports that al-Qaeda and mainstream rebel forces are largely separated from each other and control different pieces of territory. She says that the jihadists are less interested in defeating Mr. Assad than in establishing a safe haven.

That the extremists are succeeding in holding some ground has more to do with Syria’s state of war than its receptivity to Islamic fundamentalism. For the last century Syria has been a predominately secular society, with numerous religious and ethnic minorities. Many who have joined the al-Qaeda groups did so not because of their ideology, but because they were better funded and supplied. The Islamic State of Iraq depends heavily on foreign fighters. If the civil war ended, most Syrians would resist a fundamentalist regime; Ms. O’Bagy reports that there have already been demonstrations against the jihadists.

An attack that weakened regime forces could, of course, help the jihadists gain ground — but only if the United States and its allies failed to simultaneously bolster the mainstream Free Syrian Army. That’s why it is essential that Mr. Obama couple any strikes with a stepped up train-and-equip program for vetted rebel units. The way to counter the threat posed by the jihadists is not to leave the Assad regime in power, but to empower the moderate and secular majority. The U.S. failure to support moderate forces one and two years ago helped to pave the way for al-Qaeda in Syria. A congressional decision not to act now will make the jihadists stronger still.