But sending arms alone will not solve the problem. After all, small arms are already flowing to combatants from other sources in the region at an alarming rate. By more fully engaging vetted units and training them to respect the law of armed conflict, protect critical infrastructure and secure dangerous weapons sites, America can make a down payment on Syria’s future by building relationships with future partners.

In addition, the United States must take the lead in building an international consensus on what the next government of Syria will look like. We can be under no illusions: this will be very difficult and will require that we secure significant changes in policy from Russia and other countries in the region.

Establishing common cause between moderate Sunni groups and Alawites — parties that are currently at war with one another — against radical Sunni groups and Iranian proxies will be central to building Syria’s next government. The Alawites are largely clinging to Mr. Assad’s regime for fear that a Sunni victory will lead to sectarian violence against them and that it may be part of the larger, increasingly bitter Sunni-Shia divide throughout the region.

When it comes to Russia, America must display a deeper understanding of Russia’s regional interests and take advantage of our shared concerns about Islamic extremism. Russian leaders believe that Syria is becoming a safe haven for extremists, and we should take that concern seriously while at the same time insisting on sending aid to moderate groups. This could be the basis for a new understanding with Moscow and a shared approach toward Syria.

Only Russia can convince Mr. Assad that he must step aside, which is an essential first step toward a negotiated solution, and only the United States is in a position to persuade the Friends of Syria — a group of 11 nations — to isolate extremists and bring the core of the opposition to the negotiating table.

The United States must also be more aggressive in stopping Iranian support for Mr. Assad. Likewise, public and private sources of support for anti-Assad extremists in Syria should be publicized and targeted with sanctions. Other countries opposed to Mr. Assad, including American allies, must also be much more selective about who they arm and support in the war in Syria.

They must recognize that it is in their interests, as well as America’s, to build an Alawite-Sunni alliance of the center to oppose both Mr. Assad’s army and Sunni extremists with ties to Al Qaeda.