An uneasy calm hung over Beirut yesterday after Shia gunmen seized control of the mainly Muslim western part of the city from Sunni pro-government supporters. After three days of fighting, in which at least eleven people died in the debris-littered streets, the takeover by Hezbollah militants left the Lebanese capital tense and fearful that this outbreak of violence was only the precursor of a new civil war pitching militants backed by Syria and Iran against the elected Government. With the Prime Minister and other Sunni leaders besieged in their offices, government officials held an emergency meeting in the Christian heartland, while Saudi Arabia called for an urgent meeting of Arab foreign ministers to try to end a crisis exacerbating sectarian tensions across the Middle East.

Tensions have been festering since the Israeli incursion into Lebanon two years ago. Hezbollah - which led the resistance to Israel - has tried to use its subsequent brief acclaim to force the pro-Western Government to accept a dominant role for the organisation in Lebanon. When it failed, it began a campaign of violence, intimidation and assassination to subvert and overthrow the Cabinet headed by Fouad Siniora. The latest outbreak was triggered by the Government's attempt to close down a Hezbollah telephone network set up to circumvent eavesdropping by Israel and the Lebanese Government. But the weak Lebanese Army has been reluctant to enforce the law, and Hezbollah and nationalist pro-Syrian groups seized the chance to attack their rivals.

As often in Lebanon, the conflict is largely a proxy for deeper splits and tensions across the Middle East. Both Iran and Syria are trying to topple the Siniora Government. Damascus is still smarting from its forced withdrawal from Lebanon in 2005 and is determined to halt the Lebanese Government's investigation into the assassination of Rafik Hariri, the former Prime Minister, who all the evidence suggests was killed on the orders of the Syrian Government. Iran sees Hezbollah as a vital way of enforcing Shia interests, projecting its power in the Arab world and preventing any settlement with Israel.

The rest of the Arab world, in particular Egypt, Saudi Arabia and the Gulf states, insist that the Siniora Government must not be brought down by force, a position strongly backed by the West. They fear any extension of Iranian influence, and are fiercely opposed to any resurgence of Shia power, in Iraq or elsewhere. They have also been angered by Syria's intransigence, not only over the Middle East peace process, but in defying world opinion over its meddling in Lebanon.

Within Lebanon, there is widespread fear that these larger confrontations will again bring the country to its knees, as happened during the 1975-90 civil war. Already, residents are leaving, media outlets are being attacked and foreign governments, including Britain, are issuing travel warnings. For the moment, the Christians have managed to stay out of the fighting. But they, too, are divided, especially on relations with Syria. Lebanon must not again be sacrificed. The outside world, especially the West and the UN, must give unqualified support to Mr Siniora. The triumphant gunmen patrolling west Beirut must be disarmed and the city reunited. Any brief Hezbollah victory spells long-term disaster for the rest of Lebanon and the wider region.