Hizbollah is being drawn into the Syrian quagmire. as revealed by this week's reports of party members being killed fighting in the strategic Syrian town of Qusair.
Victory in Qusair is vital for the Syrian regime, as it would clear a corridor between Damascus and the coast, the stronghold of the Alawite community.
Much has been said of how Hizbollah's operations in Syria could destabilise sectarian relations in Lebanon. That worry is justified, and Hizbollah is caught between two imperatives: to bolster the regime of Bashar Al Assad, the Syrian president, or even help turn the tide of war in his favour; and to ensure that Lebanon remains calm, so that the party is not drawn into a debilitating domestic war against Sunnis.
Hizbollah believes it can reconcile these two objectives, and that ultimately the Lebanese situation is containable with the collaboration of the Lebanese army. However, the dangers are many.
In the short term, Hizbollah will want to ensure that the Qusair battle does not drag on. The party has been vital in the Syrian regime's effort to retake the area, reinforcing Mr Al Assad's ability to defend Damascus, transport weapons and men to and from the coast, and eventually perhaps even recapture lost territory in the north and north-east.
This strategy has benefited from Iranian and Russian assistance, intelligence and weapons. The prospect that Moscow will abandon Mr Al Assad in favour of a diplomatic solution today seems ludicrous. And yet this seems to be the view in Washington. The US is still focusing on the international peace conference in Geneva, and has delayed any talk of arming the Syrian rebels to prepare for the meeting.
The Russians and Iranians have a very different reading of the diplomatic track. They refuse to discuss Mr Al Assad's departure, and the offensive in Qusair suggests that they intend to help the regime reassert its military superiority before any political initiative is tabled. If Qusair falls to Mr Al Assad, then a conference would effectively serve to transform the military advances into political gains.
In this regard, Hizbollah is Iran's lever in Syria with the aim of avoiding an Assad defeat but also of preventing any diplomatic arrangement that might lead to the president's exit. The Russians have influence over Mr Al Assad, but were they to consider sacrificing him, he could always turn to Iran and Hizbollah to compensate. That may explain why Russia has supported him so completely.
It is still unclear how far Hizbollah will go on behalf of Mr Al Assad. Will it fight in areas outside Homs province and Damascus, where it is deployed today? Ideally, the party would prefer not to be caught up in an open-ended conflict - to limit its casualties, reduce sectarian pressures at home, and refocus on its paramount enemy, Israel.
This may be doable, if Qusair surrenders soon. While parts of the town have been recaptured, the rebels are putting up a ferocious fight in others. It's difficult to imagine anything other than a regime victory, since Qusair is virtually surrounded. Iran and Hizbollah have trained militias to hold the ground if they need to withdraw forces quickly back to Lebanon. The time frame and aftermath of a regime victory will define how Hizbollah's enemies react.
The Israelis are watching closely what happens in Qusair. The consolidation of a passage between the coast and Damascus would also provide geographic continuity between Alawite areas and predominantly Shia areas in Lebanon's northern Beqaa Valley, near Qusair. This could facilitate the transfer of weaponry from the regime to Hizbollah, especially in the event of a war between the party and Israel. Israel has already struck targets in Syria three times to prevent what it says were weapons shipments to Hizbollah.
More broadly, the takeover of Qusair may only harden the geographical divisions in Syria, with the regime controlling the coast, Damascus, and areas in between, while the rebels hold the rest of the country. Iran's sway on the ground could provoke the Israelis, since it means that Hizbollah has a wider area in which to function than previously. Particularly in Syria where there are no UN forces to supervise matters, Hizbollah could stockpile and transfer weapons, train combatants, and work around any eventual Israeli siege of Lebanon.
Israel can hinder this scenario, but its ability to fundamentally block it is limited. While Mr Al Assad has had close ties to Tehran for some time, he managed to maintain independence from Iran, generally keeping Syria out of fighting between Israel and Hizbollah. This will change if the Syrian leader owes his political survival to Iran. The cost could be Syria's greater integration into an Iranian-dominated security nexus in the two Arab states of the Levant.
There are fears that the loss of Qusair could provoke a violent Sunni backlash in Lebanon. Hizbollah reportedly expects car bombs to target Shia areas. The northern city of Tripoli, where Sunnis have fought Alawites sporadically for years, saw at least 10 deaths in sectarian fighting yesterday. There is speculation that Sunni gunmen may try to storm the mainly Alawite quarter, Jabal Mohsen.
To make matters worse, Lebanon is entering a political vacuum. The parliamentary elections planned for this summer are unlikely to take place, given the absence of an agreement over a new election law.
The prime minister-elect, Tammam Salam, appears far from forming a new government, owing to conflicting political demands and uncertainty over the situation in Syria.
Lebanon is used to politically tense summers, but the coming months could be the most treacherous for some time, as the country continues to suffer from the struggle next door. If Hizbollah really wants to avoid the worst, it will have to walk a tightrope successfully.
Michael Young is opinion editor of The Daily Star newspaper in Beirut.
On Twitter @BeirutCalling