Last weekend’s attack on Israel by Hamas has brought renewed scrutiny of the armed Palestinian group’s longstanding relationship with Iran, and questions about whether the Gaza-based group could have pulled off such a sophisticated and devastating operation on its own.

Iran has a long history of training and arming proxy militia groups in the region, from Gaza to Lebanon, Iraq and Syria. It supports Hamas militarily and has helped it design and produce a domestic missile and rocket system to match the capabilities and material available in Gaza — an impoverished, densely populated coastal strip that has been blockaded by Israel and Egypt for the past 16 years.

And over the past year, there have been signs that Iran and its proxies were preparing to take a more aggressive approach toward Israel.

Gen. Esmail Ghaani, who is in charge of supervising Iran’s network of proxy militias as head of the country’s paramilitary Quds Force, repeatedly traveled to Lebanon for covert sessions with leaders of Hamas and Hezbollah, a Shiite Lebanese militia that Iran also supports.

Over the past year, Mr. Ghaani worked to coordinate and unify all of Iran’s proxies, according to public statements from Iranian analysts and five Iranians familiar with the work of the country’s Islamic Revolutionary Guards Corps.

Hassan Nasrallah, the leader of Hezbollah, held an hourslong online meeting in March with an elite group of strategists from all the Iran-backed militias and told them to get ready for a war with Israel with a scope and reach — including a ground invasion — that would mark a new era, according to two participants from Iran and Syria. The participants spoke on the condition of anonymity because they were not authorized to publicly discuss the meeting.

There are conflicting accounts of whether these activities were leading specifically toward last week’s attack by Hamas, which left 1,200 Israelis dead and shattered the country’s sense of security.

Some people familiar with the operation said that a tight circle of leaders from Iran, Hezbollah and Hamas helped plan the attack starting over a year ago, trained militants and had advanced knowledge of it. That account is based on interviews with three Iranians affiliated with the Revolutionary Guards, one Iranian connected to senior leadership and a Syrian affiliated with Hezbollah.

Other people say they believe Iran had some involvement but it was not as deep. “The implementation was all Hamas, but we do not deny Iran’s help and support,” said Ali Barakeh, a senior Hamas official based in Beirut.

Iran’s supreme leader, Ayatollah Ali Khamenei, has publicly denied the country played a role, even as he and other Iranian leaders praised the carnage. “We kiss the foreheads and arms of the resourceful and intelligent designers,” Mr. Khamenei said this week in his first televised speech since the attack. But he added: “Those who say that the recent saga is the work of non-Palestinians have miscalculated.”

The United States, Israel and key regional allies have said they have not found evidence in early intelligence gathering that Iran directly helped plan the attack. The United States has collected multiple pieces of intelligence that show that key Iranian leaders were surprised by it, according to several American officials, including people who would typically be aware of operations involving the Quds Forces.

Israel has also been examining what it knew. “Israeli intelligence does not have any information according to which Iran initiated or was involved or directly assisted in the terrible attack,” said Nir Dinar, a spokesman for Israel’s military. “On the other hand, one has to be naïve to think that those in Tehran woke up on Saturday morning and were surprised to hear the news about what happened.”

It may take months or years to learn all that went into planning the attack, and why Israel’s sophisticated intelligence operation missed it. Many parties have incentives to spread disinformation or emphasize different aspects of the narrative; some may want to expand the war as others seek to limit it.

“We obviously don’t know what happened behind the scenes. This is going to be privileged, secret information that was meant to be kept secret,” said Afshon Ostovar, an expert on Iran’s military and proxies and an associate professor at Naval Postgraduate School. He added that it was safe to assume “some level of coordination,” because Iran and Lebanon would not have wanted to be caught off guard by the attack.

Hamas gunmen captured and interrogated by Israel said they had been training for the latest operation for a year, according to Israeli defense officials. Abu Ubaida, the Hamas spokesman in the Gaza Strip, said in a televised speech that the group had organized a 3,000-person battalion for the attack and had another 1,500 backup fighters. On Tuesday, Israel said it had killed close to 1,600 of those attackers.

Mr. Barakeh, the Hamas official in Beirut, said in an interview that the attack plans were so tightly held that he only found out about the assault when he received a slew of text messages early on Saturday morning.

Still, training had been taking place in Lebanon and Syria, and a secret joint command center had been set up in Beirut, according to the Iranians and the Syrian familiar with the operation.

Hezbollah’s top commandos, experienced in urban guerrilla warfare, trained Hamas members in Syria and Lebanon, according to two Iranians. Paragliders trained in Lebanon, they said, while in Syria, the Hamas members were trained to raid Israeli communities and take civilians hostage.

Hezbollah has helped train other Iran-backed militias from the region before, such as the Houthis in Yemen. The Lebanese group also deployed fighters to Syria during the war there, where they trained and fought alongside Syria’s army.

Over the past six months, Hezbollah created provocations meant to mislead and distract Israel along its northern border with Lebanon and in Syria so it would think the real threat was coming from those areas, according to two Iranians briefed after the attack.

An Israeli intelligence official confirmed that hundreds of Palestinian fighters from Hamas and other groups, and possibly even more, have been trained in Hezbollah camps in Syria and Lebanon over the past years. While they may have been trained on skills used in the recent attack, he said, that does not mean that the Iranians knew how and when they would use that training.

One of the Iranians briefed on the operation said that four days before the attack, all those participating were rounded up by Hamas commanders and isolated. Their electronic devices were confiscated and they had no contact with the outside world, something that could help explain why Israel was blindsided. On Oct. 7, several hours before launching the operation, this person said, Hamas commanders informed the attackers that they would be invading Israel on speedboats, on paragliders and over land by breaking through the barbed wire fence along the territory’s border using tractors.

This account of how the attack was carried out could not be independently verified.

In retrospect, some warning signs that Israel missed may now be more apparent.

In September, Israeli intelligence officials told The New York Times that they had intelligence suggesting Mr. Khamenei, Iran’s supreme leader, had ordered a wide campaign against Israel including targeting its citizens abroad, conducting sabotage inside its borders, and smuggling sophisticated weapons to the Palestinians to ignite a civil war in the West Bank.

That was in retaliation for shadow war operations conducted by Israel in Iran, they said.

It was not the only talk of broad action. In various meetings of Iran’s proxy militias, several attendees said, the emphasis from leaders was that it was time to take advantage of Israel’s seething internal divisions over the judicial overhaul pushed by Prime Minister Benjamin Netanyahu’s far-right coalition.

In the meeting Mr. Nasrallah held in March, he told militants to prepare for a war on a scale that would mark a turning point in the decades-long Arab-Israeli conflict (though it is unclear whether he had last week’s attack in mind).

Similarly, in audio reviewed by The Times of an April discussion among members of the Revolutionary Guards, including those involved with proxies in the region, a speaker said, “The message that is being communicated from Iran these days to the resistance is that we showcase a military maneuver to make the Zionist regime understand it is surrounded from every side.”

Even before the Hamas attack, some Israeli intelligence officials said that in hindsight, they regretted their support for Israeli targeted killings in Iran and operations to sabotage its nuclear and military facilities, because they had not been a significant deterrent to either Iran’s nuclear program or its regional activities. In fact, they had put Iran and Israel on a path of direct confrontation, one of the officials said.

On Oct. 3, four days before Hamas launched its attack on southern Israel, Mr. Khamenei’s official account in Farsi posted a message on X, formerly known as Twitter, that said: “Israel will be gone.”

The planners of the terror attack on Israel most certainly knew that it carried the risk of igniting a wider regional war. But the parties have long wanted to avenge an accumulation of grievances — from Israeli airstrikes on Gaza and Syria, to the long blockade of Gaza, to the covert war against Iran — amid a long-held collective determination to destroy Israel.

They may also have hoped to force Israel into major concessions, like lifting the blockade or keeping Israeli forces from entering the Aqsa Mosque in Jerusalem’s Old City — one of Islam’s holiest sites.

Hamas named its operation against Israel “The Aqsa Flood.”

Hwaida Saad contributed reporting from Beirut.