A year into Israel's war against Hamas, the crisis in the Middle East risks expansion. Here's how we got there

The Hamas terrorists entered Israel to kill.

The thousands who swarmed through the concrete and metal barriers separating the Gaza Strip and Israel on a Saturday last October were orchestrating the worst terrorist attack on the state of Israel since its founding.

The militants rode trucks and motorcycles and carried light arms. They shot and killed without discretion, striking civilians in their homes and vehicles. They raided a remote gathering, the Nova Music Festival, scattering the young concertgoers into hiding, massacring many of those they found, then taking others as hostages.

By the time that Saturday was over and accounted for, some 1,200 people would be declared dead, Israel later said. About 250 were taken as hostages back to Gaza.

That terror attack, which caught Israeli and international officials off-guard, ignited what has become the most-deadly conflict in the Middle East in years. Tens of thousands of people, including scores of noncombatant women and children, were killed in the first year of fighting.

"We will bring the fight to them with a might and scale that the enemy has not yet known," Israeli Prime Minister Benjamin Netanyahu said on Oct. 9.

The year of fierce and brutal fighting in Gaza between Israel and Hamas has engulfed the region, putting allies and enemies on high alert. It has reset the foundational alliances in the region, piling atop them new blood-soaked grievances.

Iran, an archenemy of the Israeli state, has continued this year backing Hamas, Lebanese Hezbollah and the Houthi rebels in Yemen, three regional nonstate groups that have each vowed to destroy Israel.

The war has proven divisive abroad, with politicians in Washington, Russia and Iran walking verbal tightropes as they voiced support for one party or another, although all have attempted to remain at arm's length from the fighting. Weapons from Iran have been used against Israel. American and European weapons have returned fire.

U.S. President Joe Biden in public comments has expressed strong support of Israel while pushing for negotiations to pause the fighting.

"President Biden reaffirmed the United States' ironclad commitment to Israel's security against all threats from Iran and its proxies, including Hamas, Hezbollah, and the Houthis," the White House said this year.

But even as he says that there exists no daylight between Israeli and American policy, he has shown signs of being frustrated with Israel -- particularly over mounting civilian casualties.

Israel has killed more than 41,000 people in Gaza, according to the strip's Hamas-run Health Ministry. Another 630 Palestinians have been killed in the West Bank, according to the United Nations.

The White House and State Department have maintained their "ironclad" backing of Israel and its right to self-defense.

"Now, that doesn't mean that every civilian death isn't a tragedy," State Department spokesperson Matthew Miller said in September. "Of course it is, and Israel needs to do everything possible to minimize civilian casualties."

The White House and State Department were busy in the last year building an international coalition with the goal of negotiating a cease-fire. And the Group of Seven said in June they were urging the parties involved to still seek a cease-fire that would include a two-state solution.

"We call on Hamas to accept this deal, that Israel is ready to move forward with, and we urge countries with influence over Hamas to help ensure that it does so," the group said in a statement.

But those and other negotiations have been time and again paused or called off.

Hamas wants a full Israeli withdrawal from Gaza. But Israel -- and Netanyahu in particular -- insists on retaining control over parts of the devastated territory. Israel is bent on decapitating Hamas even as negotiations continue. Hamas political chief Ismail Haniyeh was killed in the Iranian capital in July.

The militant wing of Hamas, which has strong ties to the group's political arm, was formed in part as a response to Israeli policy. It exists as a deadly counterweight to Israel, one with the professed goal of destroying the Israeli state.

That deadly aim was realized in stark and brutal terms on Oct. 7, 2023, as thousands of gunmen crossed the border into southern Israel. As the details of the attack spread around the world, the depths of the depravity became clear.

Israeli officials in the months afterward organized international viewings of about an hour of footage, which they said had been collected from cellphones, surveillance cameras and other sources, including body-worn cameras of Hamas militants.

An ABC News correspondent who viewed the video described the militants' deadly rampage as "chillingly unhurried" as they destroyed Israeli lives through bloody and grisly means. Garden hoes were used in decapitations. The bodies of men and women were found burned in their own homes. They had been killed indiscriminately as they lived their lives or as they attended a remote music festival.

And the most disturbing images had been scrubbed off that video, Israel said. The deaths or rapes of the youngest, most vulnerable Israelis were too gruesome to view, officials told ABC News at the time.

What followed has been an Israeli firestorm cutting its way through Gaza.

Gaza had already been crippled by a 17-year Israeli-Egyptian blockade and repeated conflicts with Israel. But the cataclysmic offensive that began in October 2023 was unprecedented in the strip's tragic history.

More than 41,000 people have been killed, according to Hamas health officials. The real number, they say, could be much higher, given the bodies of so many are thought to be trapped under rubble or missing.

Almost all residents of the strip have been displaced at least once. Swaths of Gaza have been levelled, with hospitals, schools and other key infrastructure destroyed. Disease and starvation are rampant. The flow of humanitarian aid has been choked by Israel's blockade, with many aid workers, U.N. staff, medical personnel and journalists among the dead.

Meanwhile, far-right settler groups -- among them Netanyahu's coalition partners -- are making plans to revive and expand Israeli settlements in Gaza.

For all its threats to expand geographically, the fighting for much of the year was contained within that narrow strip of land. But regional developments hinted at the wider danger. Israel, Gaza, the West Bank, Lebanon, Syria, Iraq and Iran have come under attack by enemy forces since Oct. 7.

Perhaps the most striking were the two Iranian ballistic missile attacks against Israel in April and October. Though both were largely defeated by Israel and its allies, the launches raised the risks of an extended exchange between the two enemies.

As the war with Hamas began, another longtime antagonist to Israel launched its own attacks. Hezbollah, another Iran-backed militant group, which is based in Lebanon, began cross-border aerial strikes within 24 hours of the war's beginning. Those strikes have continued throughout the year, with Hezbollah leaders vowing to continue attacking Israel until the war against Hamas is ended.

Subsequent Israeli military action in Lebanon has killed more than 2,000 people since Oct. 8, according to Lebanese authorities.

Israel's campaign of cross-border strikes and targeted killings escalated on Sept. 17 and 18, with the detonation of thousands of pagers and walkie-talkies across a 48-hour period. The operation killed at least 42 people and wounded thousands, including children. Israel was behind the attack, a source told ABC News.

On Sept. 27, Israel killed Hezbollah leader Hassan Nasrallah in a devastating series of airstrikes in Beirut's southern suburbs, using American-made bunker-busting bombs to kill Nasrallah and others while they met in an underground bunker.

An expanded nationwide airstrike campaign and ground invasion of southern Lebanon followed, with the IDF ordering residents of dozens of villages to flee north of the Awali River, some 37 miles north of the Israeli border.

Israel's actions in Lebanon prompted Iran to fire its second ballistic missile barrage at the country on Oct. 1.

As the anniversary of the Hamas terrorist attack approached, Israeli leaders were mulling how to hit back at Iran while as the U.S. tries to retain some semblance of calm.

"There is nowhere in the Middle East Israel cannot reach," Netanyahu said one day before Iran's latest missile attack.

Iran's strike, Netanyahu said the following day, was "a big mistake."

Tehran, he added, "will pay for it."

ABC News' Joe Simonetti, Matt Gutman, Sohel Uddin, Jordana Miller, Lauren Peller, Selina Wang, Alexandra Hutzler, Tom Soufi Burridge, Diaa Ostaz, Michelle Stoddart, Josh Margolin, Nasser Atta, Dana Savir, Kiara Alfonseca, Emmanuelle Saliba, Mary Kekatos, Victoria Beaule and Helena Skinner contributed to this report.