It's unclear whether Hezbollah had a role in the exchange of fire last week between the Lebanese and Israeli armies along their common border. Yet here in Beirut there is suspicion that the "Party of God" may have prompted an army officer to order his men to fire at the Israeli soldiers to reaffirm that Hezbollah alone controls Lebanon's border. The incident might be best understood as part of a power play between Syria on one side and Hezbollah and Iran on the other.

Although Damascus and Tehran are allies, Syria and the pro-Iranian Hezbollah are struggling for domination over Lebanon. It's not personal, just business.

Hezbollah's principal contract is with Tehran, and its weapons are there to retaliate for any Israeli or American attack on Iranian nuclear facilities. But the party also serves a broader purpose as an Iranian military extension into the Mediterranean. To surrender this and return to being an adjunct of Syria in Lebanon appeals little to Hezbollah Secretary General Hassan Nasrallah. It does him even less good in the eyes of his patrons, who have spent what is estimated to be hundreds of millions of dollars, if not more, to arm Hezbollah.

Meanwhile, the Syrian regime of President Bashar Assad is making a bid to reimpose its hegemony over the Lebanese, which it lost five years ago. To do that it needs to bring Hezbollah back into line with Syrian priorities—to show that Damascus, not Tehran, rules again in Beirut.

Syria's 29-year-old military presence in Lebanon came to an abrupt end following the assassination in 2005 of former Lebanese Prime Minister Rafiq Hariri. The public outcry forced the only serious suspect, Syria, to withdraw its army.

The Syrians never reconciled themselves to that departure and sought to prevent the emergence of a sovereign Lebanese state and effective government. Because Syrian soldiers and intelligence agents were no longer on the ground, the Assad regime came to rely on Hezbollah to destabilize Lebanon, handing the party, and Iran, major sway over the country's affairs.

This development so alarmed Arab states, above all Saudi Arabia, that early last year King Abdullah decided to "reconcile" with Syria after years of mutual hostility. The Saudi calculation was a cynical one: Mr. Assad would be given latitude to reassert Syrian domination over Lebanon in exchange for curbing Iran's influence here. The Saudis would press Saad Hariri, the son of Rafiq who became prime minister late last year, to mend fences with Damascus. This was a golden opportunity for Mr. Assad to reverse his 2005 Lebanese setback while earning an apparent certificate of innocence from the victim's family.

Politically dependent on the Saudi regime, Mr. Hariri had little choice but to accept. He knows who killed his father, but his most immediate foe in Lebanon is Hezbollah, and he hoped that the new rapport with Syria would allow him to counterbalance Hezbollah while buying him time to consolidate Lebanon's state institutions. Mr. Hariri's gamble may fail, but he might still have one trump to play, and it explains why Hezbollah is so nervous.

Nasrallah recently said that he believed that the special United Nations tribunal investigating the Hariri assassination was likely to indict Hezbollah members. The accusations are false, he said, the tribunal just an "Israeli project" to undermine Hezbollah. The implication was clear: Mr. Hariri must sever Lebanon's ties with the tribunal, which is a mixed Lebanese-international court. Any indictment, he knows, could weaken his party.

Nasrallah seemed doubly put out by the fact that Damascus might escape accusation. That's because Serge Brammertz, a Belgian judge who led the U.N. investigation between 2006-2008, failed to adequately explore Syria's involvement.

Mr. Brammertz's German predecessor, Detlev Mehlis, had no doubts about Syria's role and recommended the arrest of Syrian suspects. Mr. Brammertz did, however, follow up on other leads, among them telephone analyses allegedly pointing to a Hezbollah role in surveying Hariri's movements. Mr. Brammertz's Canadian successor, Daniel Bellemare, may announce these and other leads in the coming months and perhaps even indict suspects.

Mr. Assad will gain much traction from selling his return to Lebanon as a way of stifling Hezbollah. But that's an illusion. The Syrian leader will not disarm Hezbollah, nor will he break with Iran, because that would deny him the ability to exploit regional rivalries.

Even if the Hariri tribunal momentarily makes Hezbollah more pliable, the Assad regime also would like to see the investigations go away to make sure Syria will not be implicated. Yet Mr. Assad does not want an open conflict over the matter, one that might pit Mr. Hariri's Sunni community against Hezbollah's Shiite community. He prefers "Lebanese" measures to scuttle the tribunal, which he can negotiate with the Saudis or even Mr. Hariri.

At a recent summit in Beirut with King Abdullah, Mr. Assad signed a statement defending Lebanon's stability and calling for all differences to be settled within the national unity government. This was a warning to Hezbollah not to intimidate Mr. Hariri on the tribunal issue or try to bring down his government. The border clash was possibly Hezbollah's way of voicing displeasure, and a reminder to Syria that Hezbollah remains its sole military stick against Israel.

The Lebanese could be pitied for having to choose between Syria and Iran, were it not their own divisions that brought about this situation. Damascus and Tehran will probably avoid open confrontation, even as they wage an understated struggle, lately through competing Lebanese intelligence agencies. But there may yet be vigor in the fisticuffs, because neither state is timid when it comes to power games. This cannot make Lebanon any more stable or the border with Israel more secure.

Mr. Young is opinion editor of the Daily Star in Beirut, and author of the recently published "The Ghosts of Martyrs Square: An Eyewitness Account of Lebanon's Life Struggle" (Simon & Schuster).