Three months ago, I visited the flower-bedecked grave of former Lebanese Prime Minister Rafik al-Hariri in a makeshift memorial tent erected beside Beirut’s Martyrs’ Square.

I also stared at the huge crater carved out of a seaside road when a car bomb killed him and 22 other people last Valentine’s Day. The aftershocks from that car bomb haven’t yet ended. They may soon cause a massive political explosion in Damascus that will be felt all the way to Baghdad and Washington.

On Friday, a United Nations investigation into the Hariri murder, led by a German prosecutor named Detlev Mehlis, pointed the finger at top Syrian officials. Mehlis’ hefty report (www.un.org/News/dh/docs/mehlisreport) reads like a thriller. It paints a detailed picture of top Syrian and Lebanese security thugs plotting to eliminate Hariri because he wanted Syrian troops to leave his country.

One Syrian intelligence source who squealed claims the plot was hatched by the powerful brother and brother-in-law of Syrian President Bashar al-Assad. He describes alleged meetings at Damascus’ Meridien Hotel, the Presidential Palace and the office of the brother-in-law, Asef Shawkat. (I stayed at the fussily decorated Meridien in June, and now wonder if I slept next to the plotters’ meeting place.)

The case isn’t yet proved, and Mehlis has asked for more time. The report says top Syrian officials haven’t cooperated. Assad told CNN’s Christiane Amanpour last week that “Syria has nothing to do with this crime.”

The strong evidence that the Syrian regime did order the murder could shake up an already unstable region. The U.N. report guarantees that international pressure on the weak Syrian regime will be intense, and it could crumble.

But there is no opposition movement positioned to replace Assad. When I visited Syria in June and spoke to several brave dissidents, they all admitted they have little following in the country. Well-known human-rights advocate Anwar Bunni told me: “The problem is that we haven’t a real organized opposition. With no political life or civil society, (the regime) has killed all political movements.”

This reality is now recognized by the Bush administration. No one wants to see Syria implode like postwar Iraq (though a few U.S. officials still nourish hopes for speedy Syrian “regime change”). Secretary of State Condoleezza Rice talks of pressing Assad for “policy change” rather than “regime change.”

Yet Assad seems unwilling or unable to make the changes he needs to deal with new realities in the region. U.S. officials express frustration at his unwillingness to shut down the flow of Arab Islamists into Iraq and his help to hard-line Palestinian groups and Hezbollah. The U.N. report on Hariri will prompt a push soon for economic sanctions against Syria at the Security Council.

If Assad fails to respond, there may be pressure within the Bush administration for military action inside Syria along its border with Iraq. This would be counterproductive, alienating Sunni tribes that straddle the Syria-Iraq border; those tribes might become even more willing to aid the flow of Islamists.

The Syrian leader could have taken pre-emptive action to arrest officials involved in the Hariri murder. Instead, his regime stonewalled U.N. investigators.

He may be too weak to act, too beholden to members of his family clique who allegedly ordered the murder. He may believe his weakness will save him – because others fear post-Assad chaos. Or he may think there’s nothing he can do that will satisfy the Bush team.

Whatever the reason, the Syrian leader can dally no longer. The U.N. report on Hariri’s death has hastened Assad’s moment of truth.