After a half-year of fighting and a final lightning advance, Libyan rebels streamed into the capital Tripoli on Sunday night, greeted by eruptions of euphoria and amazing scenes of jubilant Libyans tearing down posters of the “Supreme Guide of the Revolution,” the longest reigning dictator in the Arab world.The astonishing death throes of Muammar Gaddafi’s more than four-decade rule testify to the struggle of the Libyan people, whose courage in the face of tyranny we applaud, but also to the leadership of British Prime Minister David Cameron, French President Nicolas Sarkozy, and NATO head Anders Fogh Rasmussen who showed faith in the tenacity of the rebels and helped avert an atrocity. Those who favored humanitarian intervention have been resoundingly vindicated by the rebel victory.As the regime draws ever closer to final collapse, these leaders’ principle of intervention comes out looking far better in retrospect than the “lead from behind” doctrine of the Obama administration, which was slow in responding to the crisis, unenthusiastic and irresolute once it did engage, and wished as quickly as possible to hand off the mission to its European allies.But as the US learned the hard way in Iraq and Afghanistan, toppling a repressive regime is by no means the end of the story. As a new beginning in Lybia is contemplated, a different question urgently presses itself on the international community. In a New York Times op-ed about the European Union’s options in Libya, Catherine Ashton, the EU’s high representative for foreign affairs and security policy, noted that “sometimes the toughest question in world politics is: ‘And then what?’” Reconstruction will be fraught with challenges, and will first of all have to attend to the dangers of political, tribal, ethnic and regional fragmentation. Feuding among the rebels already led to the killing of their military commander, Abdul Fattah Younes, last month.The long-suppressed Muslim Brotherhood may vie for supremacy not only with Libya’s principal tribes but with disaffected non-Arab ethnic groups in the South, including the Berbers, Tuareg and Toubou.In forging a united Libya eager to reenter the international community after being pariahs for so many decades, the National Transitional Council (NTC), the rebel governing body, has little to work with. Libya lacks both the functioning institutions of civil society (Gaddafi explicitly tried since 1969 to create a state without institutions) and an officer corps (which he deliberately weakened beginning in the early 1990s).So if democracy is to replace dictatorship, if Libya is not to suffer the fate of Iraq or Afghanistan, NATO must work with the NTC to lead an international post- Gaddafi stabilization effort, to prevent a descent into the chaos of intertribal fighting and warlordism, to prevent Gaddafi loyalists from exploiting the power vacuum to mount an insurgency, to minimize vengeful violence against regime supporters, and to secure intelligence records, the Central Bank and vital oil installations.It is imperative, too, that NATO buttress the NTC’s authority and credibility as it works to build security institutions that are answerable to civil authorities.The transitional government will also require help in addressing the dire needs of the more than one million refugees who have fled to Tunisia (some 575,000 refugees), to Egypt (an estimated 356,000), and other neighboring states.Some steps can be taken immediately. Muammar Gaddafi; his sons Seif Islam and Captain Khamis Gaddafi, who heads the 32nd Brigade, the regime’s best-trained force; and his brother-in-law, the head of Libyan military intelligence Abdullah Sanousi, all deserve to be put on trial and made to answer for their crimes against the Libyan people. The US should deploy a staff of post-conflict experts as soon as possible to Tripoli. And Abdel Basset Muhammad Megrahi, sentenced to life in prison for his role in the bombing of Pan Am Flight 103 in 1988, ought to be recaptured and sent back to prison.Finally, there is one more despot to be deposed. Let us hope that the Libyan message – and of the renewed vigor of the Arab Spring – is received loud and clear by Bashar Assad of Syria, who only Sunday adamantly dismissed American and European calls for him to step down as “meaningless.” His turn has come.