SYRIA’S CIVIL war has become a wretchedly complicated problem. As the parties prepare to meet in Geneva for the second round of United Nations-sponsored peace talks, the government has launched vicious barrel-bomb attacks on Aleppo and other cities; more moderate Islamist rebel groups, including the Free Syrian Army, are openly at war with Al Qaeda affiliates; and Al Qaeda-linked groups are now fighting among themselves.Meanwhile, the war’s spillover effects are worsening. The fighting has heightened instability in the region; US and European citizens are streaming into Syria to take up jihad; and there is a growing consensus that the post-World War I Middle East boundaries are coming undone. Indeed, the viability of Syria, a multi-ethnic state, is being threatened by multiple armed groups supported by external sponsors – Iran, Saudi Arabia, Qatar, Russia, the United States, Turkey, France, and many private donors – who themselves have conflicting aims.Here are three ways to simplify the equation and maximize the chances that the parties to the Geneva II peace conference will be able to agree on more than the desirability of someday holding a Geneva III.First, the most important contribution that this conference can make to the possibility of a negotiated settlement and a political transition in Syria is to change the principal parties’ incentives. In the run-up to Geneva II, each party has sought to strengthen its hand at the negotiating table by killing as many adversaries and holding or regaining as much ground as possible. The task now for would-be peace brokers is to halt that dynamic by agreeing on criteria for participation in whatever elections will eventually be held, regardless of whether President Bashar al-Assad remains in power until then.Those criteria must include the parties’ willingness to allow humanitarian aid to flow to all Syrian civilians under their control and an end to war crimes and crimes against humanity, including systematic targeting of medical personnel, starvation of populations under siege, and executions of war prisoners. Here the UN must reaffirm its "responsibility to protect” doctrine, not as a justification for military intervention, but as a fundamental principle agreed by all countries: governments must protect their citizens. If Assad’s Ba’ath party cannot uphold that responsibility, it forfeits its own legitimacy as a participant in any future government.Second, the international community must re-establish the basis for its engagement. When the Syrian conflict began, it was an internal matter, with UN involvement limited to humanitarian and refugee issues. But now the conflict has spread across the Middle East, destabilizing Lebanon and Jordan and threatening to fracture Iraq. The UN Security Council is charged with addressing breaches or threats to international peace, a criterion that is now clearly met.As a permanent member of the Security Council, Russia thus has an obligation to act; it (and China) can no longer hide behind the argument that the UN should not be engaged in Syria’s internal affairs. At a time when the Winter Olympic Games in Sochi put Russia squarely on the international stage, the US and other Security Council members should plan a series of resolutions that confront the Kremlin with the choice of meeting its responsibility or applying its own leverage to bring the conflict to an end.Finally, the single most important step that US President Barack Obama could take is to put the credible threat of force back on the table. In three years of increasingly bloody conflict, the only diplomatic success was achieved when Assad believed that he faced US missile strikes. He suddenly saw the desirability of getting rid of his chemical weapons.But most experts believe that military force is off the table. The US public sharply rejected Obama’s planned missile strikes to punish Assad for the repeated use of chemical weapons, and a recent Pew poll indicates that a majority of Americans believe that the US "should mind its own business internationally and let other countries get along the best they can on their own.”Obama’s job, however, is to look beyond opinion polls, particularly when it comes to safeguarding national security. The US has withdrawn from Afghanistan and Iraq, but the gains so painfully won are being reversed. Al Qaeda is back and is fighting for its own proto-state in western Iraq and eastern Syria, which is far closer to Europe and the US than the caves of Afghanistan.Perhaps Obama thinks that he or his successor can deal with that threat down the road. If Al Qaeda operatives begin threatening the US from the Islamic State of Iraq and Syria, the US will just take them out with drones, as it has done in Afghanistan, Pakistan, and Yemen. But if he is willing to contemplate using force against Al Qaeda without international authorization in the future, why not use drones now to strengthen the moderate Syrian opposition and force Assad into serious negotiations?The threat of cruise-missile strikes last September was enough to send Al Qaeda members in Syria scrambling for the hills. A strike designed to destroy Assad’s air force and prevent him from dropping bombs full of nails on his people would concentrate his mind on a diplomatic solution.Obama should announce that the US is committed to a political solution in Syria, and that his government will do whatever it can to bring about such a solution through next week’s peace conference and follow-up action. But if a ceasefire has not been achieved in the next three months, the US should work with regional organizations and all friends of the Syrian people to authorize a set of military strikes on Al Qaeda-linked forces and on the killing machine that Assad’s government has aimed at civilians.The Obama administration should make the case for this to the American people in terms of straightforward US security interests. After all, if Geneva II fails, Geneva III will not be about Syria alone. It will be about how to end a war raging across the entire Middle East.Anne-Marie Slaughter is a former director of policy planning in the US State Department (2009-2011), is President and CEO of the New America FoundationCopyright project-syndicate