As Israel and the Gaza-based Palestinian militant group Hamas wage war, worries are building that the conflict will escalate to engulf the wider Middle East. Of particular concern is the role of Israeli foe Iran.

Iran provides money and weapons to Hamas, which the US and European Union have designated a terrorist group, as well as Hezbollah, the Lebanese militia that regularly clashes with Israel. For years, Israel and Iran have engaged in a shadow war, quietly attacking each other on land, by air and at sea — in Iran’s case sometimes by proxy. They have mostly sought to avoid open clashes that would risk provoking all-out war, but that danger has grown.

1. How has Iran responded to the Israel-Hamas war?

Iranian officials celebrated the attack that set off the war, in which Hamas fighters from Gaza overran Israeli military bases and villages, killing 1,300 people and taking hostage about 200. Iranian Supreme Leader Ali Khamenei said “we kiss the hands of those who planned the attack.” Iran has denied involvement in the assault, and US and Israeli officials have said they have no direct evidence that it played an active role. Iran has vowed that Israeli reprisal attacks, which have killed thousands of people in Gaza, will be answered by Iran’s so-called axis of resistance — which includes anti-Israeli and anti-Western groups in Lebanon, Syria, Iraq, Afghanistan and Yemen. Iranian Foreign Minister Hossein Amir-Abdollahian warned Oct. 16 that the expansion of the Israel-Hamas war to multiple fronts was “approaching the inevitable stage.” He called for an oil embargo of Israel, which relies on Iran for none of its crude supplies and on Middle East producers for a very small portion.

2. Why are Iran and Israel enemies?

They were allies starting in the 1950s during the reign of Iran’s last monarch, Shah Mohammad Reza Pahlavi, but the friendship abruptly ended with the Islamic revolution in Iran 1979. The country’s new leaders cut ties with Israel and adopted a strong stance against the Jewish state, decrying it as an imperialist power in the Middle East and supporting groups that regularly fight it. Israel regards Iran’s potential to build nuclear weapons as a threat to its existence and is thought to be behind a campaign of sabotage against the country’s atomic program. Iran’s leaders say they have no ambition to build nuclear arms. The Israelis point to a cache of documents their intelligence spirited out of Iran in 2018 that suggests otherwise. Israeli officials have repeatedly implied that if Iran were to reach the brink of weapons capability, they would attack its nuclear program using air power, as they did Iraq’s in 1981 and Syria’s in 2007.

3. What is Hezbollah?

In 1982, Israel invaded its northern neighbor, Lebanon, in an effort to destroy Palestinian guerrilla bases there. In response, a militia that would become Hezbollah was formed by Lebanese Muslims belonging to the Shiite branch of Islam dominant in Iran. Their group to some extent became a proxy for Iran’s premier security force, the Islamic Revolutionary Guard Corps. In 2006, Israel and Hezbollah fought a major war, in which Israeli air strikes inflicted massive damage on Lebanon’s infrastructure. Since then, the two sides have periodically struck at one another. Such hostilities increased in the wake of the new Israel-Hamas war. Hezbollah is thought to have an arsenal of more than 100,000 rockets and missiles, some of which can reach central Israel, where Tel Aviv is located. Israel moved to solidify defenses on its border with Lebanon as well as Syria.

4. What’s the concern about Syria?

A new battleground for Israel and Iran opened up in 2011, when Syria erupted in civil war. Through the course of the war, Iran built up a military presence in the country to support its ally, President Bashar al-Assad, and to facilitate the transfer by land of weaponry meant for Hezbollah from Iran to Lebanon via Iraq and Syria. In an effort to stop the arms flow and counter this second hostile presence on its northern border, Israel has carried out hundreds of strikes inside Syria against arms shipments and other targets it says are linked to Iran and its allies. Some Iranians have died, according to media accounts.

5. What’s happened at sea?

The year 2019 marked the beginning of tit-for-tat attacks on commercial vessels, part of a larger disruption in shipping in waterways that are critical to the movement of much of the world’s waterborne oil. Although neither Israel nor Iran, a major oil and gas producer, has accepted responsibility for the hits on ships connected to the other, they are widely thought to be behind them. Loss of life has been rare, but in July 2021, a British and a Romanian crew member were killed when an Israeli-operated ship was struck in the Gulf of Oman by a drone that US officials linked to Iran. Previous targets have included Iranian tankers carrying oil destined for Syria; an Iranian ship off the coast of Yemen that served as a floating base for the Revolutionary Guards; and cargo ships belonging to or linked to Israelis.

6. What about attacks inside the two countries?

Though Iran has mostly absorbed Israeli strikes on its interests in Syria, in 2018 its forces there fired a barrage of missiles toward Israeli positions in the Golan Heights, a plateau Israel captured from Syria in the 1967 war and later annexed. Israel replied with a much greater show of force. For its part, Israel is widely thought to be behind the assassination in Tehran of five Iranian nuclear scientists since 2010 and several attacks on nuclear sites inside Iran. In April 2021, Iran blamed Israel and vowed revenge for an explosion at its largest uranium enrichment facility in Natanz, which it said caused significant damage to its centrifuges. It was the second time in less than a year that the site had been hit by a suspicious blast. Israel neither confirmed nor denied it was responsible for either attack. In October 2021, an Iranian general said Israel was likely behind a cyberattack that paralyzed gas stations across Iran. And in January, after an Iranian ammunition depot near the central city of Isfahan was attacked in a drone strike, two US newspapers reported that Israel was responsible.