The tide has turned against Syrian president Bashar al-Assad's regime. Although rebels are battling for control of the capital, Damascus, and the second city, Aleppo, civil war having come to the dictator's door is a turning point from which there can never be reconciliation. He has reneged on promises to create a unity government and refused to go into exile, but the thousands of people killed by his army make continued rule, even if opponents are defeated, impossible. For the good of Syria and the region, it is time for him to go quietly. Assad has vowed not to give in, putting faith in the might of the helicopter gunships, tanks and weapons supplied by unswerving ally, Russia. They continue to pound positions taken by rebels of the Free Syrian Army, boosting a death toll that human rights monitors put at more than 19,000 since the uprising began 17 months ago. But for every citizen that the regime kills, for each home that is flattened, ever more opponents are created. Last Wednesday's bomb blast in Damascus that gouged at the heart of the military command, killing four top security officials, among them the defence minister and the president's powerful brother-in-law, was an ominous sign that the 40-year-old regime had lost its grip. Rising numbers of officials and generals are defecting. Rebels have seized city districts and border crossings. To outsiders, it seems a replay of the events of the past year-and-a-half in Egypt, Libya, Tunisia and Yemen that led to the removal of often brutal dictators and autocrats. The circumstances are similar, but not the same - Syria, bordering Iraq, Israel, Jordan, Lebanon and Turkey, is a decidedly more complex and potentially dangerous situation. Syria has chemical and biological weapons and Assad is so desperate to cling to power that there are fears he will use them against his own people. Diverse ethnic and religious divisions open the threat of sectarian bloodletting, as in neighbouring Iraq. Tides of refugees are already flooding the borders, but that could only worsen were there to be post-Assad uncertainty. Iran, Assad's best regional friend, could turn the conflict into a full-fledged Middle Eastern war. For these reasons, the world has to come together and start planning what is to follow. Former UN secretary-general Kofi Annan's peace plan based on diplomacy was wise, but has been overtaken by events. Sanctions will achieve little; China and Russia have been right to use their power of veto against the idea in the UN Security Council. Moscow has to cut Assad loose and the US, despite a presidential election looming, has to be ready to lead nations in providing funding and aid for any new government. Neighbours and the Arab League have to ensure regional stability. Peacekeepers may be needed. But before all that, Assad has to be realistic and for Syria's sake, accept one of the offers to go into exile.