BEIRUT, Lebanon — Everywhere in the Middle East these days, people are muttering about the possibility of war: between Israel and Hamas, or Israel and Hezbollah, or Israel and Syria, or among bickering Lebanese factions. Or maybe this war will involve everyone.

What might set off such a catastrophic conflict? Maybe it starts with Israeli soldiers trying to trim a tree.

That’s exactly what happened on Tuesday, when Lebanese troops fired on Israeli forces who were pruning a tree along the border between the two countries. That set off a series of skirmishes that killed two Lebanese soldiers, a Lebanese journalist and an Israeli commander.

This clash, the most serious in four years, underscores why Lebanon’s southern frontier with Israel is the most volatile border in the Middle East today, and how easily a confrontation could spiral out of control. Western policymakers must not shift their attention away from Lebanon, a small country that has long been the staging ground of proxy wars in the region.

The latest fighting did not involve Hezbollah, the Shiite political party and militia that has fought Israel for decades. But Hezbollah remains a central player in the dangerous drama that is unfolding along the Lebanese-Israeli border. When a pro-American coalition won Lebanon’s parliamentary elections last year, a seductive conventional wisdom emerged in the West: Because Hezbollah and its allies were defeated at the polls, the group would lose some of its luster and a U.S.-backed government would rule Lebanon. In fact, Hezbollah remains the country’s dominant military and political force. It holds the key to both domestic and external stability, and its actions will help determine whether there is another war with Israel, or if Lebanon will once again be wracked by internal conflict.

In November, the U.S.-backed Sunni leader Saad Hariri was chosen as prime minister after he agreed to share power with Hezbollah and its allies. But Hariri’s government has no influence over the militia and its weapons buildup along the border. As long as the Lebanese Army remains weak, Hezbollah can argue that its fighters are needed to defend the country against Israel.

When Lebanon’s 15-year civil war ended in 1990, all of the country’s militias were disarmed. But the government allowed Hezbollah to keep its weapons as “national resistance” against the Israeli occupation of southern Lebanon, which continued until May 2000. After the Israelis withdrew, many Lebanese asked why the group did not disarm and become a strictly political movement. Hezbollah insisted that its mission of resistance was not over because Israel was still occupying a strip of land — called Shebaa Farms — at the murky intersection of Israel, Syria and Lebanon. (The United Nations later determined that the area is Syrian territory, not Lebanese.)

In July 2006, Hezbollah abducted two Israeli soldiers in a cross-border raid, setting off a 34-day war that crippled Lebanon’s infrastructure, displaced one million people, and killed more than 1,200 Lebanese, the majority of them civilians. Since that conflict ended, both sides have been preparing for a new round. Hezbollah leaders boast that the group now has an even larger and more potent cache of missiles than it did four years ago. Israeli officials, who have also escalated their war rhetoric in recent months, estimate Hezbollah’s arsenal at between 40,000 and 80,000 rockets.

The basic problem is that Hezbollah sets its own military strategy and it makes decisions that could lead to war without the involvement of the Lebanese state. Israeli Prime Minister Benjamin Netanyahu has threatened to hold the Lebanese government responsible for the militia’s actions. That puts Hariri in an extremely difficult position and it will make him reliant on the Obama administration to keep Israel at bay.

The border has flared up several times over the past year: two suspected Hezbollah weapons caches mysteriously exploded, and Al Qaeda-linked groups were blamed for two salvos of rocket fire into Israel from southern Lebanon. Under the United Nations Security Council resolution that ended the 2006 war, U.N. peacekeepers are supposed to intercept illegal weapons shipments and raid storage sites south of the Litani River. They have rarely done so. While Hezbollah continues its arms buildup, Israel has also violated the U.N. resolution with frequent overflights into Lebanese airspace and by planting surveillance devices on Lebanese territory.

Neither Israel nor Hezbollah has an immediate interest in starting a war. Israel is more concerned right now about Iran, although if Israel attacks Iran’s nuclear facilities, the Shiite militia would likely be part of the Iranian retaliation. As part of Lebanon’s new government, Hezbollah cannot afford to instigate another war with Israel. But the danger of heightened rhetoric and a military buildup is that minor incidents along the border could spiral out of control.

By engaging Israeli troops this week, the Lebanese Army was trying to assert government authority over the border. The army had not been in control of the southern border since the late 1960s, and it only deployed there after the 2006 war. But the army’s action is largely symbolic because Hezbollah effectively controls the frontier.

Still, the symbolism was not lost on Hezbollah’s leader, Sayyid Hassan Nasrallah, who quickly tried to portray the conflict with Israel as nationalist struggle in which his militia and the Lebanese Army are partners. “The army guards the resistance, and the resistance guards the army,” he said at a rally in southern Beirut on Tuesday night. “The resistance will cut off any Israeli hand that tries to harm the Lebanese Army.”

Nasrallah confirmed what most Lebanese already knew: Without a strong central state that can defend itself, Hezbollah remains the most powerful force in Lebanon — and its weapons guarantee that dominance.

Mohamad Bazzi is a journalism professor at New York University and an adjunct senior fellow for Middle East studies at the Council on Foreign Relations.