Stephen J. Hadley was national security adviser in the George W. Bush administration.

The Arab Awakening has caused a crisis in the Middle East that will take years to sort out. There is one Middle East crisis, however, that must be resolved in months, not years.

Every American committed to preventing Iran from obtaining a nuclear weapon should urge Congress to grant President Obama authority to use military force against the Assad regime in Syria.

The inauguration of Hassan Rouhani, Iran’s new president, offers some hope of a diplomatic settlement that eliminates the prospect of a nuclear-armed Iran. But Rouhani will need the approval of Iran’s supreme leader, Ali Khamenei, who has made confrontation with the United States the centerpiece of his rule. Only internal pressure and the threat of U.S. military action will cause Khamenei to accept a nuclear deal. That is why Presidents Obama, Bush and Clinton have emphasized that all options — including the use of military force — are on the table.

Bashar al-Assad has repeatedly and flagrantly crossed a U.S. “red line” by using chemical weapons against his own people. If the United States does not take military action, how credible will be the U.S. threat to use military force if the Iranian regime continues to pursue nuclear weapons? If that threat is not credible, then only months from now our nation could face the prospect of accepting a ­nuclear-armed Iran or having to resort to military force to prevent it.

If the Obama administration had aggressively trained and armed moderate elements of the Syrian opposition during the past 18 months, our country probably would not have to contemplate military action now. But one does not have to concur with past Syria policy to agree that, once established, U.S. red lines cannot be crossed without consequence.

Enforcing the red line against chemical weapons use is not the only U.S. national interest at stake in Syria. There is the moral claim presented by a humanitarian disaster that has killed more than 100,000 people, created more than 2 million refugees and internally displaced an additional 4 millionpeople. Refugee flows and the increasingly sectarian conflict could destabilize Lebanon, Iraq, Jordan and Turkey — creating a regional war pitting Sunni against Shiite. There is the real prospect of al-Qaeda establishing a haven in Syria from which to launch attacks against U.S. allies — and Americans. Or Iran and its Hezbollah allies could succeed in saving the Assad regime and establishing a broader hegemony in the Middle East, thereby threatening U.S. allies, including Israel.

To protect these U.S. interests, U.S. military action in Syria must go beyond a few missile strikes designed to deter or degrade future chemical weapons use. It must be robust enough to erode the Syrian regime’s military advantage.

This does not mean U.S. troops on the ground or a Kosovo-style weeks-long bombing campaign. But it does mean hitting aircraft, missile and artillery systems and their bases — the weapons Assad uses to cause mass casualties among innocents.

Responsible people have called for diplomacy and negotiations, but those have been tried and failed. Assad is not willing to negotiate because he is too strong, the opposition because it is too weak.

A negotiated outcome becomes possible only by changing the balance of power: a limited U.S. strike that sets back Assad’s military, followed by the training and arming of those vetted elements of the opposition that support a democratic, inclusive Syria and oppose al-Qaeda and other extremists.

Congress should adopt legislation that authorizes and urges the Obama administration to pursue such a policy.

The goal is fracturing the Syrian regime so political and military elements of the regime can join with moderate and democratic elements of the opposition to establish an interim government that can begin to wind down the war, protect all Syrians (including Alawites and other minorities) and go after al-Qaeda.

Such an outcome will be hard to achieve, and it will take time. The alternatives — an Assad military victory, a takeover by al-Qaeda and other extremists, a prolonged stalemate or descent into chaos — all threaten U.S. interests.

Having asked Congress for authorization, it will be virtually impossible for President Obama to take military action if Congress votes no. To mitigate the adverse consequences of a “no” vote, the administration would have to take bold action. Most are steps the administration should take anyway, but after a rebuke from Congress, they would be even more important.

In Syria, this means expanding and accelerating the training and arming of moderate, democratic elements and the provision of humanitarian assistance, especially to areas liberated from the regime. In Afghanistan, this means leaving a significant U.S. military force after 2014 to help stabilize the country and check Iranian activity. In Iraq, this means shoring up U.S. support for the government as it battles a resurgent al-Qaeda and seeks to resist Iranian pressure. In the region, this means strengthening diplomatic and security support to allies nervous about Iranian hegemony.

Otherwise, lack of U.S. military action in Syria will be read as a victory by Iran — making a nuclear-armed Iran and Iranian hegemony more likely.