Hezbollah is both a militant group and political party that controls much of southern Lebanon.
It is considered a terrorist organization by the United States, although the European Union has only blacklisted its armed wing.
The Shi'ite group emerged during the chaos of Lebanon's civil war and in response to Israel's invasion in 1982.
Hezbollah is believed to have been created by Iran's powerful Islamic Revolutionary Guards Corps (IRGC), the elite branch of the country's armed forces.
The aim of Shi'a-majority Iran was to unite Lebanon's Shi'ite political groups and militias under one organization, says Danny Citrinowicz, a research fellow at the Israel-based Institute for National Security Studies.
Hezbollah was also intended to be a key part of Tehran's deterrence strategy.
"Iran sees the organization as the main factor that will deter Israel or the U.S. from going to war against Iran and works tirelessly to build the organization's power," Citrinowicz said.
The U.S. State Department says Iran has armed and trained Hezbollah fighters and injected hundreds of millions of dollars into the group. The State Department has previously described Hezbollah as "the most technically capable terrorist group in the world."
Hezbollah has around 40,000 fighters, according to the office of the U.S. director of national intelligence, and is said to have over 150,000 rockets and missiles at its disposal, including weapons that can penetrate deep into Israeli territory.
The Israel Defense Forces said Hezbollah has turned itself "into the world's most powerful nonstate actor."
Iranian Supreme Leader Ayatollah Ali Khamenei (right) greets Hezbollah Secretary-General Hassan Nasrallah (left) at his office in Tehran in July 2000.
Citrinowicz says Iran may not dictate orders to the organization but Tehran "profoundly influences" its decision-making process.
He describes Hezbollah not as a proxy but "an Iranian partner managing Tehran's Middle East strategy."
Hezbollah is a key part of Iran's so-called axis of resistance, a loose alliance of proxies and Tehran-backed militant groups against Israel and the United States.
Led by Hassan Nasrallah, Hezbollah has developed close ties with other Iranian proxies and Tehran-backed armed groups, helping to train and arm their fighters.
Citrinowicz said Tehran "almost depends" on the Lebanese group to oversee its relations with other groups in the axis of resistance.
Hezbollah's kidnapping of two Israeli soldiers in 2006 triggered a 34-day conflict with Israel, which launched air strikes and a ground offensive into Lebanon.
The war, which ended after a UN-brokered cease-fire, was a major boost to Hezbollah's political standing in Lebanon and the region.
While Hezbollah remains a politically influential player in Lebanon, its support base does not extend beyond the country's Shi'ite community.
Many members of Lebanon's Christian, Druze, and Sunni Muslim communities accuse Hezbollah of operating outside the state's control and of dragging the country into regional conflicts.
"Hezbollah doesn't enjoy the same level of popularity and public support that it did back in 2006," said Hamidreza Azizi, a fellow at the German Institute for International and Security Affairs. He attributes the decline in Hezbollah's popularity to a series of economic and political scandals involving the group in recent years.
Hezbollah has suffered major setbacks in recent months, with Israel killing many of its senior commanders, neutralizing a significant part of its military arsenal, and disrupting its communications.
While the group has been weakened, experts say the Lebanese militant group should not be written off.
"Hezbollah has enough resources and experience to inflict considerable damage to Israel in a ground war," Azizi added, noting that the group is better armed and trained than the U.S.- and EU-designated Palestinian terrorist group Hamas.