An international tribunal has called on Lebanon’s government to arrest four suspects indicted in the 2005 car-bomb killing of former Prime Minister Rafik Hariri and 21 others. All four of the accused are members of Hezbollah, the militant Shiite movement whose cynical mix of politics and armed intimidation helped bring Prime Minister Najib Mikati to power.

Hezbollah’s leader, Hassan Nasrallah, vows the four will never be turned over for trial. Mr. Mikati has a legal duty to arrest them. He claims to be politically independent of Hezbollah. This is the perfect chance to prove it.

Carrying out these arrests will require extraordinary political courage. Failure to do so would cost Lebanon dearly, threatening its civil peace and leaving no doubt that the real powers in Lebanon are Hezbollah and its backers in Syria.

The reverberations of Mr. Hariri’s murder have rocked Lebanon for the past six years, threatening its recovery from the disastrous 1975-1990 civil war. Mr. Hariri was a symbol of that recovery. Popular fury over his killing helped force out Syrian occupation troops. But the failure to bring his killers to justice has revived the bitter mistrust among Lebanon’s main confessional groups — Sunnis, Shiites, Christians and Druse — opening the way for Hezbollah’s alarming rise.

Other countries have important roles to play in urging Mr. Mikati to turn over the indicted suspects. That goes especially for members of the United Nations Security Council, whose authority stands behind the international tribunal. The United States, the European Union and Russia have issued helpful statements. Saudi Arabia and Egypt have spoken less emphatically. All need to raise their voices in the weeks ahead.

Lebanon has until early August to arrest the four men. If it does not, the international tribunal has the authority to try them in absentia. If convicted, they would become international outlaws, subject to future arrest and punishment. Any Lebanese government that continued to shelter them could be subjected to international sanctions, including suspensions of economic assistance. Lebanon should do its duty now, and arrest the four indicted suspects and turn them over to the tribunal.