Charles Dunne is director of Middle East programs at Freedom House. David J. Kramer is president of the group, and William H. Taft IV is chairman of its board.

United Nations special envoy Kofi Annan decried Friday the massacre of more than 200 people in Tremseh, Syria, thought to be the worst single incident since the demonstrations began in the spring of 2011. But what else will be done? Many arguments have been advanced against a more robust U.S. response to the crisis, including:

●We do not know enough about the Syrian opposition and military insurgency;

●What follows might be worse;

● Past interventions didn’t go well; and

●We can’t intervene everywhere.

Not one of these arguments stands up to moral or geopolitical scrutiny.

So far, more than 17,000 people have been killed, many of them in indiscriminate attacks on towns by the Syrian army or in massacres of civilians by Syrian security forces and their allied “shabiha” militias. Syria is rapidly descending into a civil war that could lead to ethnic cleansing along the lines of Iraq in 2006. That would have serious consequences for regional stability. Iran and Russia are already militarily involved, whether through boots on the ground (Iran) or major arms sales (Russia). China, worried about interference in human rights abusers’ internal affairs, has joined Russia in protecting Damascus in the U.N. Security Council.

On the other side, Saudi Arabia and Qatar are arming and financing the military opposition, primarily to eliminate another Shiite regime allied with Tehran and help shape a Sunni fundamentalist follow-on government. Syria’s military opposition is increasingly gaining ground, the Institute for the Study of War has found. The Annan plan for Syria’s future — which advocates, among other things, a “national unity government” that neither side will accept — was dead even before it was not adopted at recent international meetings. Both sides know their survival is at stake and will not yield.

With Syria’s future already taking shape, the question is: Will the United States play a role in shaping it? Or will it stand aside and let those less committed to democratic principles do it?

The United States can’t afford to stay on the sidelines. A failed state in Syria is likely to spill over into Iraq and Lebanon and spur debilitating refugee flows to Turkey and other neighbors. It will intensify a proxy war between Saudi Arabia, its Gulf allies and Iran. A Syrian collapse would create a fundamentalist threat to Israel’s sense of security and heighten the danger of miscalculation or conflict.

But this crisis also presents opportunities. The destruction of the Assad regime — which may be weakening, as military defections, including that of Gen. Manaf Tlas, son of the regime’s former defense minister, increase — would raise the prospect of another country moving toward democracy in the heart of the Middle East. Removing a key ally from Iran’s grasp could tip the balance of power in Lebanon and weaken the Iranian leadership. And breaking the Tehran-Damascus alliance on Iraq’s east and west borders might assist Iraq in its struggle toward democracy.

The Obama administration should throw in its lot more firmly with the opposition, both civilian and military. It needs to stop talking about what it won’t do and start discussing what it might do differently to end the bloodshed. This will make the regime’s downfall — and a transition to a stable, more democratic country — more likely.

First, the administration should dispense with its informal contacts with the main opposition umbrella group, the Syrian National Congress, and formally recognize and work with the SNC as the transitional authority. This would boost the SNC’s capacity to speak effectively to the outside world and plan for Syria’s future. In exchange, the administration should insist that the SNC build bridges to minorities such as Syria’s Christians and Alawites, who, fearing what might follow Assad’s Alawite regime, have continued to support the government.

Similarly, Washington should publicly help the Free Syrian Army, the Turkey-based military opposition organization, coordinate with military elements in Syria, particularly the regional military councils. The administration must play a more active role in coordinating arms deliveries from third countries to ensure they reach secular elements of the opposition who will not turn on us after they win. The United States should also provide its own arms, training and intelligence, helping to ensure that we become a sought-after partner, with commensurate influence.

The White House should publicly consider enforcing humanitarian corridors (“no drive” zones) as well as no-fly zones to counter the regime’s increasing use of helicopter gunships. It should launch formal discussions of such measures with NATO allies. Merely planning for serious military options would have an important psychological effect on the regime and its military forces, possibly prodding more defections.

Lessons of things not to do when we intervene have been learned the hard way in Iraq and Afghanistan. Srebrenica and Rwanda have provided their own hard lessons — most important, the cost in lives and U.S. moral standing — for failing to intervene. The United States must summon its leadership skills and, as it did in Libya, put an end to a disastrous conflict that challenges our sense of ourselves as Americans as well as our national interests.