DESPITE LEBANON'S remarkable popular uprising earlier this year and the subsequent departure of Syria's occupation army, progress toward a new and freer political order has been halting in recent months. That's one reason why this month's arrests of four senior security officials on murder charges was important: It dealt a severe blow to those, including Syria and its proxies, who still seek to thwart the Lebanese political opening through terrorism. For much of the Arab Middle East, the step toward solving the assassination of former prime minister Rafiq Hariri was groundbreaking in another respect. For once, the perpetrators of an act of political violence could be clearly identified and held responsible, along with their state sponsors.

The men arrested include the most powerful actors in the Syrian puppet regime that controlled Lebanon until a few months ago: the former chief of general security, police chief and military intelligence chief, and the current commander of the Republican Guard brigade. All were the collaborators of President Emile Lahoud, another Syrian stooge who remains in office but faces growing pressure to resign.

Lebanon's crisis began a year ago when Syria's strongman, Bashar Assad, ordered the Lebanese parliament to extend Mr. Lahoud's tenure in violation of the constitution. Mr. Hariri, who had led Lebanon's economic comeback from years of civil war, protested -- and was directly threatened by Mr. Assad. After he moved toward supporting an opposition coalition in parliamentary elections, a massive explosion destroyed his car.

Though almost all the Lebanese who took to the streets en masse after the Feb. 14 murder blamed Syria, few believed the case would be solved. That it has come, at least, to arrests is the result of diligent work by Detlev Mehlis, a German investigator appointed by the United Nations to investigate the assassination. Mr. Mehlis gathered evidence that he shared with Lebanese judicial authorities, who acted with the encouragement of the newly elected government. Mr. Mehlis, in turn, has been able to count on strong Security Council support thanks to close cooperation between the United States and France. A rare accord between the Bush administration and the government of Jacques Chirac has demonstrated how much the transatlantic allies can accomplish when they work together in the Middle East.

Mr. Mehlis isn't finished yet: He says he wants to interview several senior Syrian officials. After first denying him access, Mr. Assad has reluctantly invited the U.N. envoy to Damascus, again because of Western pressure. The Syrian leader is likely to be shunned by Western leaders at the U.N. General Assembly this week; if Mr. Mehlis can provide details of Syria's involvement in the Hariri killing, Mr. Assad could find himself in the pariah's box once occupied by Libya's Moammar Gaddafi, who suffered years of international sanctions because of his sponsorship of the 1988 Lockerbie bombing of an American airliner. Mr. Mehlis has already done Lebanon a great service; we wish him good luck in Damascus.