At midnight on the Syrian-Israeli border on May 8–9, 2018 a multiple-rocket launcher system operated by the Quds force—an expeditionary special forces unit of the Iranian Revolution Guard Corps—fired a salvo of twenty unguided 333mm Fajr-5 rockets towards Israel. (You can see the apparent rocket launch here.) Four of the rockets were shot down by Israeli Iron Dome air defense system and the rest missed and landed in Syrian territory.

A few hours later, around ten Israeli surface-to-surface missile launchers and twenty-eight F-15I and F-16I jets unleashed seventy cruise missiles and precision-guided glide bombs that struck Iranian logistical bases and outposts throughout Syria. The Iranian rocket launcher was destroyed, and when Syrian air defenses attempted to engage the Israeli fighters, five batteries were knocked out.

The May 9 clash is considered the first direct clash between Iranian and Israeli forces, an event likely linked to the Washington’s withdrawal from the Iran nuclear deal the day before the attack. However, observers of the region might recall that Israeli warplanes had struck an Iranian convoy in Syria earlier that same day. There were additional strikes on May 6 and April 29 that killed scores of Syrian and Iranian troops—possibly including an Iranian general—and knocked out an S-200 surface-to-air missile battery.

Recommended: This Is How China Would Invade Taiwan (And How to Stop It).

Recommended: North Korea’s Most Lethal Weapon Isn’t Nukes.

Recommended: 5 Worst Guns Ever Made.

By one count, there have been over 150 Israeli strikes in Syria stretching all the way back to 2012. Many of the raids have targeted transfers of advanced weapon systems to Hezbollah, or been made in response to cross-border attacks.

If you connect the dots, it becomes clear that the May 9 exchange was the most overt flare-up of a long-running proxy war. Iranian and Hezbollah troops—having effectively secured the government of Bashar al-Assad from possible overthrow—are busily establishing a long-term presence and transferring advanced weapons into Syria and Hezbollah, including drones, surface-to-air missile systems and rocket artillery. At the same time, Israeli warplanes are attempting to destroy these sites and weapon systems before they get deeply entrenched. The U.S. exit from the nuclear deal appears to have prompted Tehran to finally authorize a direct retaliatory attack on Israel—even if it was an ineffective one.

It’s unusual for two regional powers without a common a border to be at each other’s throats. However, a series of historical circumstances have brought them to more more open military conflict than ever before.

Syria, Lebanon and Hezbollah

Under the regime of the Shah, Iran developed relatively close economic and military ties to Israel. The Iranian Revolution in 1979 brought hardline clerics to power and a formal end to diplomatic ties, but Israel continued to supply Iran with over $500 million in desperately needed weapons during the bloody Iran-Iraq War. At the time, the Israeli government saw Iraq as a more proximate threat—mainly due to its nuclear weapons and ballistic missile programs.

However, the seeds of a deeper Iranian-Israeli conflict were sown in the civil war in Lebanon. The creation of Israel in 1948—and its conquest of additional territories in 1967—displaced hundreds of thousands of Palestinian refugees. Unable to return home, the Palestinian refugees became a permanent, nationless population in neighboring Arab countries. Their presence in the small, multicultural state of Lebanon eventually destabilized a precarious balance of power between diverse factions divided by ethnicity, religion and ideology, contributing to the outbreak of a civil war in 1975.

Seven years later, Israeli troops entered Lebanon in Operation Peace for Galilee, an effort to tilt the war in the favor of Christian factions in Lebanon. Syria—which had fought multiple wars with Israel for control of the Golan Heights—had already deployed troops in Lebanon in support of Palestinian factions, so Syrian tanks and jet fighters clashed with Israeli forces in massive battles. President Ronald Reagan also dispatched troops to Lebanon in an attempt to influence the conflict, but withdrawn them in October 1983 after a truck bombing killed 241 Americans.

Meanwhile, religious divisions facilitated Iran’s involvement in the conflict. The most important schisms in the Islamic faith lies between the Sunni and Shia branches of Islam, comparable to the Catholic-Protestant divide of Christianity. Iran—the chief Shia country in the Islamic world—organized and armed a coalition of Shia fighters called Hezbollah to fight the Israelis.

Though Hezbollah initially opposed Syrian influence in Lebanon, Damascus would eventually become a junior-partner in the management of Hezbollah. While Syria has a majority Sunni population, the ruling Assad family were Alawites—a minority associated with Shia Islam—which may have contributed to warm Iranian-Syrian ties.

Israel eventually withdrew from the Lebanese quagmire, and the war ground to its conclusion in 1990 with the defeat of the Maronite Christian faction, and the restoration of a tenuous multiparty democratic system—in which Hezbollah was entrenched as a key player. The Shia group became an odd combination of political party, de facto regional government in southern Lebanon, standing army and international terrorist group (with its sights set on the Israeli forces on the Lebanese border).

Tehran and Damascus also used Hezbollah as a lever to influence Lebanese politics. For example, Syria and Hezbollah are generally believed to be culpable in the assassinations of two-time Lebanese prime minister Rafic Hariri in 2006 using a 4,000-pound truck bomb.

Washington Clears the Way for Tehran’s Rise as Regional Power

During the 1990s, Iran ramped up anti-Israel rhetoric to cultivate political support in the wider Muslim world. However, in 2003, the moderate government of Mohammad Khatami transmitted a wide-ranging peace offer to restore relations with the United States and Israel. However, the Bush administration did not bother replying to a members of the ‘Axis of Evil.’

In April of the same year, U.S. forces invaded Iran’s old enemy, Iraq. It may be difficult to recall today, but in the weeks following the fall of Baghdad, many in Washington openly speculated that Tehran or Damascus might be the next to fall.

However, Iraq had been a Shia-majority nation ruled by a Sunni dictator. With the overthrow of Saddam Hussein, Iraqi Shia politicians and clerics ascended to power and soon shifted the nation towards warmer relations with Tehran. Ironically, the United States had removed one of the chief geographic barriers to Iranian influence. Shia repression of their erstwhile Sunni persecutors would also inspire radical Sunni groups (such as ISIS), prompting the formation of violent, Iran-backed Shia militias to deal with them in a vicious sectarian cycle.

In 2005, the hardline Mahmoud Ahmadinejad was elected president of Iran. A zealous ideologue, Ahmadinejad combined demagogic anti-Israel rhetoric and pageantry—for example, he sponsored an international Holocaust-denial conference—with increased military support and training for Hezbollah. Tehran also began sponsoring Hamas, a Sunni Islamist group that eventually secured control of the Palestinian Gaza Strip territory in 2007.

In the summer of 2006, this dynamic escalated into the 2006 Lebanon War, when Israel—in response to an ambush in which two Israeli soldiers were kidnapped—began a large-scale bombing campaign targeting Hezbollah forces in Lebanon, later escalating into an invasion.

The sheer scale of the bombardment reportedly stunned Hezbollah’s leadership—as well as Lebanese civilians. (The Israel Defense Forces estimates it killed 600 to 700 Hezbollah fighters in the war, while most sources put the death toll in both combatants and noncombatants in Lebanon at around 1,100.) However, Russian-built Kornet-E and Metis anti-tank missiles and a dense network of defensive positions allowed Hezbollah fighters—backed up by Iranian Revolutionary Guard personnel—to inflict unexpectedly heavy losses on Israeli tanks and infantry.

The war ended in a ceasefire and clashes diminished in frequency, in part due to a successful surge in from UN peacekeeping forces. Hezbollah and Israel licked their wounds and were soon were engaged in other conflicts.

Israel was also concerned with Iran and Syria’s covert nuclear weapons program, and Iran’s growing ballistic missile capabilities in particular. In Syria, Iranian, Syrian and North Korean engineers had been collaborating on a nuclear reactor facility and a new chemical weapons plant. However, in July 2007 the al-Safir chemical plant suffered a ‘mysterious’ explosion, while Israeli warplanes destroyed the reactor in Deir-ez-Zor that September, effectively bringing an end to Syria’s nuclear ambitions.

Iran, however, lies beyond the easy striking distance of Israeli jets, with several intervening international borders. Israel therefore began lobbying Washington to launch a preventive war to knock out the Iranian research program. Though this idea was popular with neoconservatives in the Bush administration, the Iraq War by then had proven such a debacle that political support was lacking.

Instead, the Israeli military employed clandestine means to strike at Tehran’s nuclear program. Starting in 2010, Israeli assassination attempts killed at least five and wounded one Iranian nuclear scientist. Iranian attempts to retaliate—via international terror attacks—were mostly unsuccessful.

Iran, Hezbollah and Russia Team Up to Save Assad

In 2011, the Arab Spring caused a wave of political unrest to sweep across the Middle East, igniting a civil war in a drought-stricken Syria. By 2012, the Damascus had lost control of vast swathes of the country to a disunited Sunni Arab and Kurdish rebel groups.