Hezbollah, which was founded in the 1980s during a civil war and an Israeli invasion, is now the country’s dominant political and military force. It is unrealistic of Saudi leaders and the Trump administration to expect that it can be supplanted by a popular Lebanese groundswell against it or removed by a foreign military force without causing catastrophic damage to Lebanon.

Saudi Arabia’s new ruler, King Salman, and his son and designated heir, Crown Prince Mohammed bin Salman, are pursuing a far more aggressive foreign policy than previous Saudi rulers. They have been bolstered in this by Mr. Trump’s support for the kingdom in its conflict with Iran. Now that Iran’s ally, Mr. Assad, has essentially won the civil war in Syria, Saudi Arabia is looking to contain Iranian influence elsewhere. Lebanon is a tempting target.

The Saudis have also been emboldened by their recent outreach to Shiite factions in Iraq, especially the nationalist cleric Moktada al-Sadr, who visited the kingdom in July and met with Prince Mohammed. The Saudis are hoping to cultivate Mr. Sadr and other Shiite leaders who can be a counterweight to Iranian influence in Iraq, especially ahead of parliamentary elections next year.

But the Saudis won’t be able to find a Sadr in Lebanon, a political figure who can offer a serious alternative to Hezbollah and Iranian influence in the Shiite community.

Since the end of Lebanon’s civil war in 1990, Hezbollah has entrenched itself in the largely Shiite areas of southern Beirut and southern Lebanon. With Iranian support, it opened schools and hospitals, provided business loans and fielded candidates for parliamentary elections. It also extended its military capability, deploying thousands of missiles along the border with Israel.

In February 2005, Rafik Hariri, a billionaire construction tycoon and Lebanon’s former prime minister, was assassinated in a bombing in Beirut. His death deprived Lebanon of its most prominent Sunni leader — and Saudi Arabia lost its most important Lebanese ally. After Mr. Hariri’s death, his son Saad took over his father’s Saudi-based construction empire and the Sunni political mantle in Lebanon.