The return of Sunni anchor to Beirut or rather Saad Hariri's visit on Friday, his first since 2011, comes after open conflict between the army and jihadists on the border with Syria that has killed 17 troops and left 19 kidnapped. Hours after his arrival, a military source said troops had begun entering the restive town of Arsal for the first time since the fighting erupted a week ago.
Hariri has said Saudi Arabia, one of his chief allies, had pledged $1 billion to shore up the army and security forces against jihadists.
His return to Lebanon also underscores the seriousness of the clashes in the Arsal region in eastern Lebanon on the Syrian border.
Fighting that began there on August 1 has eased, and a military source said Friday night that the army has "started to enter" Arsal, setting up a checkpoint and advancing slowly.
But earlier, residents who tried to return to their homes were fired on by snipers and it remained unclear whether gunmen had withdrawn from the town.
Sunni clerics have mediated a truce under which the militants agreed to return to Syria and talks are continuing over the release of 17 policemen and 19 soldiers being held hostage.
The violence in Arsal is the worst in the border region since the Syrian war began in March 2011, and has raised further concerns about the effects of the conflict on Lebanon. Despite Beirut's effort to insulate itself from the war next door, the fighting has spilled over and stoked existing political and sectarian tensions in Lebanon.
Much of Lebanon's Sunni community, including Hariri, supports the Sunni-dominated uprising against President Bashar al-Assad. But many Lebanese Shiites support Assad, and the powerful Shiite Hezbollah movement has sent fighters to bolster his troops against the uprising.
Hariri, 44, has voiced unconditional support for Lebanon's army in the fight against jihadists in Arsal, calling it a "red line". But some of his constituents accuse the army of allowing Hezbollah free rein to fight in Syria and failing to protect Sunnis.
The fighting in Arsal erupted last Saturday afternoon, when gunmen attacked soldiers after the arrest of a Syrian man accused of belonging to Al-Qaeda's Syrian affiliate Al-Nusra Front. At least 17 soldiers have been killed, and 19 remain kidnapped along with 17 policemen.
Dozens of militants have been killed, and field hospitals have reported at least 47 deaths, with more than 250 wounded.
The fighting prompted army chief General Jean Kahwaji to urge France to speed up delivery of weapons being bought under a $3 billion deal financed by Saudi Arabia.
And Hariri announced on Wednesday that the kingdom had pledged $1 billion in funds to bolster the Lebanese security forces.
On Friday, Hariri said his stay in Lebanon would be "long", and that the visit came "after the Saudi donation, which we must examine how to implement and translate in support of the army".
Analysts said he would seek to rally the Sunni community around the army and bolster his standing in Lebanon, which he left in 2011 after his government collapsed when ministers from Hezbollah, his chief political rival, resigned.
"Hariri's return can be seen as an attempt at unifying the Sunni community both around Lebanese state institutions, particularly the army, as well as around Saudi Arabia, which is now championing itself as a major counter-terrorism force in the region," said Lina Khatib, director of the Carnegie Middle East Centre.
"Such a rallying of Sunnis would have a stabilising effect on the country and detach the Sunni community from being attracted to the extremism presented by groups" such as the Islamic State, she said.
Khatib added that Hariri was also "striving to regain a central political standing in Lebanon as the country looks forward to holding parliamentary and presidential elections".
Hariri's father, former prime minister Rafik Hariri, was assassinated in a 2005 Beirut attack that his supporters blame on the Syrian regime and its Lebanese ally Hezbollah.