For the last two years, there has been bitter debate in Washington over what the United States should do in Syria.

Beneath the surface, though, there has been broad agreement on what should not happen: President Bashar al-Assad crushing the rebels, remaining in power and handing Tehran a strategic victory that boosts its influence across the region would be a destructive setback.

President Obama’s decision to arm Syria’s rebels is a basic first step toward slowing Assad’s advance and Tehran’s rise. Fears of another Iraq are understandable. So is criticism that gradual assistance is too little too late.

But along with the humanitarian argument for assistance there is a growing strategic justification for the United States to act.

As the conflict destabilizes surrounding countries, sending limited military assistance to the rebels is the best of several bad choices in Syria.

Over the last several months, vast military assistance from Iran and Hezbollah has accelerated the killing and shifted the military balance in Assad’s favor.

The outright defeat of the rebels would inflame Shiite-Sunni tensions across the Middle East, tilt the regional balance of power toward Tehran and weaken America’s Arab allies.

“If Iran wins this conflict and the Syrian regime survives, Iran’s interventionist policy will become wider,” Mustafa Alani, director of security and defense at the Dubai-based Gulf Research Council told the Washington Post. “Its credibility will be enhanced.”

In Lebanon, Hezbollah’s all-out support for Assad is fueling growing rage among Sunnis. In Iraq, Shiite-Sunni tensions are soaring as well. Saudi Arabia’s leading religious authority recently urged Sunnis to fight in Syria. Al-Qaeda chief Ayman al-Zawahiri did as well.

Frustrated longtime American allies in the region call the administration’s response to the conflict feckless. Jordan and Saudi Arabia recently cut the United States out of a new rebel training program, The New York Times reported. The United Arab Emirates refused to host a meeting of regional defense officials to discuss how to help Syria’s rebels. They said the gathering would descend into bickering without strong American leadership.

Obama’s decision to send small arms and ammunition to the Syrian opposition changes that dynamic.

But the White House should move cautiously. It should resist calls to deploy U.S. ground troops or create safe havens and no-fly zones. Instead, it should first see if Gen. Selim Idris, chief of staff of the Free Syrian Army, can deliver on his promise to build more effective rebel units and decrease the role of jihadist fighters.

American military aid should be limited, escalated gradually and used as a lever to increase the chances of a diplomatic settlement. If Idris makes headway, his forces should also receive sophisticated anti-tank weapons. Anti-aircraft missiles, however, should remain off the table. The risk of jihadists obtaining them is too high.

If Idris fails to make headway, Washington should decrease its support.

Syria’s rebels — not American troops — must change the military balance on the ground. If the opposition remains divided and dominated by jihadists, the United States should accept that it cannot succeed without an effective partner on the ground.

For understandable reasons, any American involvement in Syria is hugely unpopular in the United States. The overwhelming sentiment is “stay out,” in the wake of Iraq and Afghanistan. But let’s be honest. Fears of a terrorist attack in the United States, as well as Washington’s close alliance with Israel, enmesh us deeply in the region.

In an effort to block sophisticated weapons from falling into the hands of jihadists, Washington has restricted the type of arms provided by Turkey, Saudi Arabia and Qatar, according to rebel leaders. They said the United States halted deliveries after videos appeared in March showing jihadists using anti-tank weapons from Saudi Arabia.

The United States should maintain its alliance with Israel. But we are kidding ourselves if we think $3 billion in annual aid to Israel, combined with sweeping diplomatic support, makes us “neutral” in the Middle East. From ending Iran’s nuclear program, to countering Hezbollah, to billions in annual U.S. aid to Egypt as mandated by the Camp David agreement, the United States is — and should be — deeply engaged in securing Israel.

It may be too late to stop the centrifugal forces in Syria. Arming the rebels now may make no difference. Sectarianism may still consume the region. The powerful dynamics unleashed by the post-Arab Spring are still playing out.

But viewing every situation as another Iraq is not productive. The United States’ options go beyond doing nothing in Syria or launching a ground invasion. Arming one side in a conflict — as occurred in Bosnia — can help produce a diplomatic settlement.

Crudely speaking, three forces are in play across the Middle East. Sunni jihadists, who are bent on forcibly implementing hardline Islamic law; Shia fundamentalists, who are holding Iran’s staged elections Friday, and the broad, inchoate group of moderates who embrace modernity, individual rights and basic democracy.

We focus too much on the fundamentalists and too little on the moderates. As I’ve written before, the United States should view these moderates as allies, listen to their ideas on how we can strengthen them and act where possible.

Responding to the pleas of moderate Syrian commanders for weaponry is one small step in that direction. Now, the rebels must unify and seize their opportunity.

Ideals aside, if Washington stands by while Assad, Hezbollah and Tehran crush the rebels, it will embolden our enemies. Arming the rebels may be too little, too late. But for strategic, not simply humanitarian reasons, Obama should be praised for doing it.

Maine native David Rohde is a columnist for Reuters, a two-time winner of the Pulitzer Prize, and a former reporter for The New York Times and Christian Science Monitor.