To Get a Truce, Be Ready to Escalate FOLLOWING the Obama administration's conclusion last week that President Bashar al-Assad's forces have used chemical weapons, the talk in Washington is all about military assistance to Syria's rebels. That aid is necessary, but observers have overlooked a crucial point: the American decision to give rebels lethal aid, though it might eventually contribute to the overthrow of Mr. Assad, opens an opportunity for concerted diplomacy to end the bloodshed. President Obama's decision to supply small arms and ammunition to the rebels is a step, possibly just the first, toward direct American intervention. It raises risks for all parties, and especially for Mr. Assad, who knows that he cannot prevail, even with Russian and Iranian military aid, if the United States becomes fully engaged. We used a similar strategy against the Serbian leader Slobodan Milosevic in Kosovo in 1999, where I commanded American forces, and showed that NATO had the resolve to escalate. With a brutal dictator like Mr. Assad, only the knowledge that he cannot prevail will force him to negotiate an exit. Mr. Obama has sought a diplomatic solution for some time, but has been reluctant to take steps that might lead to military intervention. Rightly so. No one wants more death and disruption in the Middle East, nor another open-ended military commitment - and certainly not the Pentagon. Despite the humanitarian tragedy in Syria, most of the conditions that have allowed previous interventions to succeed are absent. Legal authorization from the United Nations is unlikely, given opposition from Russia and China. Syria's rebels are fragmented politically and militarily; some are religious extremists with professed ties to Al Qaeda. What would follow Mr. Assad's departure is unclear, which is why he has managed to retain support from Shiites and other minorities, besides his own Alawite sect, who fear the consequences of a Sunni-led takeover. Iranian agents, along with their allies from Hezbollah, are involved, as are the Russians, who have a naval port at Tartus. But inaction is not an option. The bloodletting - more than 90,000 are estimated to have died so far - has deepened the region's longstanding Shiite-Sunni struggle. It has become a proxy war, with Sunni Arab states backed by the West, like Saudi Arabia and Qatar, challenging Iran's reach to the Mediterranean via a proxy, Hezbollah, and Syria. The risk of going beyond lethal aid to establishing a no-fly zone to keep Mr. Assad's planes grounded or safe zones to protect refugees - options under consideration in Washington - is that we would find it hard to pull back if our side began losing. Given the rebels" major recent setbacks, can we rule out using air power or sending in ground troops? Yet the sum total of risks - higher oil prices, a widening war - also provide Syria (and its patrons, Iran and Russia) a motive to negotiate. If Mr. Obama can convince Iran that he is serious, and is ready to back up his new promise of aid with additional forces, Iran and Russia will know the risks: Mr. Assad could lose his regime, and most likely his life. Higher oil prices would cost China, which has blocked anti-Syrian initiatives at the United Nations, dearly. In 1999 in Kosovo, the West used force as leverage for diplomacy. There, a limited NATO air campaign began after diplomatic talks failed to halt Serbian ethnic cleansing. The bombing lasted 72 days, and plans for a ground invasion of Serbia were under way when Mr. Milosevic finally bowed to the inevitable. Of course, the Middle East is not the Balkans, the Russian government is more confident now than it was then, and Americans are tired after a decade of war. But there are similarities: The Kosovars, too, bickered among themselves, and some were said to be terrorists. The Russians backed Serbia - and at one point suggested that their naval fleet in the Black Sea would intervene. Like Mr. Assad, Mr. Milosevic was rational and calculating - he, too, wanted to survive. Mr. Assad knows that Mr. Obama can be surprisingly resolute, as in his approval of drone strikes and the military operation to kill Osama bin Laden. While the United States begins to supply the rebels, there is a crucial opening for talks. Russia or China could recalculate and help lead Syria to a real peace process, as Viktor S. Chernomyrdin, a former Russian prime minister, did in Kosovo in 1999. Iran could emerge from a truce with Hezbollah's power in Lebanon (and its strong links to Iran) intact. The formula for diplomacy is clear: a cease-fire agreement; a United Nations presence; departure of foreign fighters; disarmament of Syrian fighters; international supervision of Syria's military; a peaceful exit for Mr. Assad, his family and key supporters; a transitional government; and plans for a new Syria. The conflict, and the diplomacy needed to end it, are likely to play out simultaneously. All parties will be recalculating their options and risks, so any assurance Mr. Obama gives Americans that he will limit our engagement would reduce the chances of success. This is a nerve-racking time, but the consequences of inaction are too high. Working together, America, Russia and China can halt Syria's agony and the slide toward wider conflict. Mr. Obama's decision might be the catalyst to get that done.