On Wednesday night, President Obama called on Congress to authorize arms for moderate Syrian rebels, calling them the "best counterweight to extremists like ISIL." This is an argument I have been making to policymakers since December 2012 based on my own trips to rebel areas. But certain policymakers have preferred to delay action indefinitely due to "questions surrounding the composition and goals of Syrian opposition groups." Even after the president's speech, House Middle East Subcommittee Chair Ileana Ros-Lehtinen stated, "the situation on the ground is so fractured and too complex."

Congress members like Ros-Lehtinen need to start tuning in to events on the ground. On Tuesday, over 40 leaders from the Syrian rebel group Ahrar al-Sham were massacred at a meeting, depriving Ahrar al-Sham of its entire top leadership. The Islamic State in Iraq and Syria (ISIS) reveled at news of the deaths, as Ahrar al-Sham has always been in ISIS's crosshairs. Since Ahrar al-Sham is among the strongest rebel groups, Tuesday's attack could decisively tilt the Syrian battlefield in ISIS's favor. ISIS might soon place a death grip on the rebel stronghold of Aleppo City, which is twice the size of Mosul.

Ahrar al-Sham is often considered the most hardline Syrian rebel group, and analysts who wish to scare policymakers away from the rebels often point to the group's "extremist discourse". In the past, there has even been talk of designating Ahrar al-Sham a terrorist organization. But actions speak louder than words. Ahrar al-Sham's record shows that there is a clear line between even the "extremist" rebels and ISIS, and that denying rebels aid because of a "fractured and too complex" Syrian battlefield is quite foolhardy.

When the Syrian opposition captured its first provincial capital in March 2013, under a coalition led by Ahrar al-Sham, the city saw a flowering of civil society activity that included Armenian Christians in leadership roles. On January 1, 2014, when ISIS killed Ahrar al-Sham figure Hussein al-Suleiman, it sparked a mass anti-ISIS uprising that removed ISIS from northwestern Syria and nearly brought down ISIS's international headquarters. When ISIS counterattacked to fend off the assault, and carried out its worst massacre anywhere in Syria until last month, most of the victims were from Ahrar al-Sham.

I do not mention these events to indicate my agreement with Ahrar al-Sham's political ideology. There are other rebel groups far more aligned with American vaues and with the initial democratic values of the protests. But there is a reason that Syrians initiated a revolt specifically against ISIS, and not "extremist" rebel groups like Ahrar al-Sham. Of all the fighting groups in Syria, only ISIS is hell-bent on establishing an international Islamic state. In April 2013, when ISIS was formed, Ahrar al-Sham immediately rejected the move, warning, "Realize the magnitude of the event, the danger of regionalizing the conflict in this way." Within five weeks, ISIS had pushed Ahrar al-Sham out of the rebel-held provincial capital of Raqqah and begun executing activists.

Today, the world realizes what Ahrar al-Sham and all other Syrian rebels realized long ago: that the formation of ISIS was a seminal event with regional implications, and that the group must be confronted. As late as March 2014, ISIS controlled almost none of the Syrian territory from which it stormed Mosul. That territory was safe in the hands of Syrian rebel groups, including numerous "extremist" factions. But just five days after ISIS consolidated control of this border territory, Mosul fell. That is the difference between ISIS and "extremist" rebels like Ahrar al-Sham.

Two weeks ago, Syrian Al-Qaeda affiliate Jabhat al-Nusra kidnapped 45 U.N. peacekeepers near the Syrian-Israeli ceasefire line. Rebels quickly denounced this move as "perfidy," and I reached out to various rebel leaders to facilitate the peacekeepers' release. When I spoke to Ahrar al-Sham leader Hasan Aboud, he replied "What a foolish move!" and pushed for the peacekeepers' release. Aboud was killed in Tuesday's massacre, but all of the peacekeepers were released two days after his death.

Even if some American politicians are confused, the American people seem to understand the difference between ISIS and "extremist" rebels. According to an NBC News/Wall Street Journal poll released Wednesday, Americans feel less safe now than at any time since 9/11. They also believe, by a substantial majority, that the U.S. should strike ISIS sanctuaries inside Syria. Americans never felt this insecure when Ahrar al-Sham or other "extremist" rebels established safe havens inside Syria. This is because when Syrian rebels--whatever their political beliefs--conquer territory from Assad or ISIS, they do so to seek greater opportunity and freedoms for their homeland.

Syrian dictator Bashar al-Assad desires only to maintain his manic grip on power, and ISIS leader Abu Bakr al-Bagdadi is fixated on enacting his twisted vision of a global caliphate. Syrians discovered very quickly that neither Assad nor Bagdadi could provide them a decent future. That is why, while protests against rebel corruption do occur, only ISIS and Assad have felt the full fury of popular uprisings and insurgencies against them.

The well-being of Americans and Syrians is now intertwined. If ISIS or Assad are able to capture Aleppo City, the main rebel stronghold, thousands of Syrians could die in the ensuing massacre and it could prove a fatal blow to Syrian rebels. But then Americans would also suffer a calamity. The Pentagon has assessed that airstrikes only "stall" ISIS advances but can not eradicate ISIS sanctuaries. Syrian rebels are the only ground forces in Syria with a record of rolling back ISIS. If Syria's rebels were to disappear, Americans would have no ground partners left in Syria. They would be forced to choose: accept permanent ISIS sanctuaries in Syria, or send ground troops back to the region.

Americans should trust Syria's rebels, because they alone can ensure that Americans never face such a grim decision. It is a great shame that, over the past two years, Americans stood by for fear of rebel groups like Ahrar al-Sham as Syrians were slaughtered by the thousands. But it is not too late. President Obama should make good on his vow to "conduct a systematic campaign of airstrikes" against ISIS by ordering immediate airstrikes to defend Aleppo. Congress should speedily pass the President's authorization request for an arm-and-equip program for moderate rebels.

I lived in Syria for nearly three decades and have visited Syria's liberated areas. Contrary to some news reports, rebel fighters are not barbarians. By and large, Syrians of all political ideologies are civilized people facing two savage war machines. If America acts to defend Syrians from their attackers, a new partnership between Americans and Syrians can begin today.

Ghanem is the senior political adviser and government relations director for the Syrian American Council in Washington, a board member of the Coalition for a Democratic Syria, and a fellow at the Syrian Center for Political and Strategic Studies. Follow @MhdAGhanem