When Hamas militants launched a deadly cross-border attack on October 7, 2023, they launched a war with Israel that devastated Gaza. They also set off shockwaves that reshaped the Middle East in unexpected ways.

Powerful alliances are broken. The long-established “red lines” have been crossed. The decades-old dictatorship in the heart of the region has been swept away.

Fifteen months after the October attacks, with a ceasefire agreement between Israel and Hamas set to begin on Sunday, here’s a look at how the region has radically transformed.

Israel

Israel has reasserted its military dominance, but may face heavy diplomatic and domestic costs.

The country’s leaders have treated the attacks led by Hamas as an existential threat and are determined to defeat Hamas and weaken its main backer, Iran. Israel not only managed to weaken Hamas in Gaza, but also decimated the Lebanese Shiite group Hezbollah and dealt a heavy blow to Iran’s network of Middle Eastern allies.

Closer to home and in the realm of global public opinion, Israel’s successes were more ambiguous. Although his attack on Gaza severely weakened Hamas, it did not destroy it, as the government had promised to do.

Israel’s economy has been battered by the war, and the country’s polarized politics – briefly ignored when the war began – appear to have returned to their fractious state of affairs. The country’s international position has been damaged, which threatens its diplomatic goals, such as the normalization of relations with Saudi Arabia.

That dynamic could change again with the inauguration on Monday of President-elect Donald J. Trump, who in his first term sought to normalize ties between Arab states and Israel and may try to revive those efforts.

In the long term, it is difficult to predict what threats Israel may face from a generation of young Lebanese and Palestinians who have been traumatized by the death and destruction that Israeli bombing has caused in their families and homes.

Hamas

Hamas and its leader at the time of the October 7 attack, Yahya Sinwar, wanted to start a wider regional war between Israel and Hamas’ allies. But the group failed to predict how the conflict might end.

For Palestinian civilians, the future looks bleaker than ever.

Israel’s bombardment and invasion forced nearly all Gazans from their homes and killed more than 45,000 people, according to Gaza health authorities, who do not distinguish between civilians and combatants. Israel has turned huge parts of the enclave into ruins.

Israel killed Mr. Sinwar and the rest of Hamas’s military and political elite, and the group’s popularity among Gazans has faded, although American officials assessment that Hamas recruited almost as many fighters as it lost during the 15 months of fighting.

And yet its remaining leaders can claim its survival as a victory.

Israel insists that Hamas cannot rule the enclave after the war, but has resisted calls to draw up a plan for a post-war Gaza. Gulf states such as Saudi Arabia now say they will not normalize relations with Israel unless it commits to the establishment of a Palestinian state.

Lebanon

A shattered Hezbollah, once the jewel in the crown of Iran’s so-called axis of resistance, has loosened its grip on Lebanon. But Israel’s invasion and bombing has left Lebanon facing billions of dollars in reconstruction costs amid the economic crisis that preceded the war.

Hezbollah, once Lebanon’s dominant political and military force, has seen a major upheaval since the 2023 attack. Israel has killed most of its top leaders, including Hassan Nasrallah. Its patron Iran is weakened. And its supply lines through Syria are at risk. More broadly, the group’s fundamental promise to Lebanon – that it alone can protect the country from Israel – has been reversed.

Years of political gridlock, largely blamed on the militant group, eased enough this month to allow Lebanon’s parliament to elect a new president and name a prime minister backed by the United States and Saudi Arabia.

Despite the strikes, Hezbollah can still call on thousands of fighters and has the support of Lebanon’s large Shiite Muslim community. Perhaps they will still find a way to rebuild within Lebanon’s factional political system.

Syria

The overthrow of Bashar al-Assad last month – one of the most dramatic and unexpected consequences of October 7 – dismantled a brutal authoritarian regime. But the inevitable turmoil that followed created the conditions for new power struggles.

Almost 13 years Mr. al-Assad has largely contained a rebellion against his family’s five-decade hold on power – with the help of Russia, Hezbollah and Iran.

But with Moscow focused on its war in Ukraine and Iran and Hezbollah reeling from Israeli attacks, rebels led by the Turkish-backed Hayat Tahrir al-Sham Islamists sensed an opportunity. They swept through Syria and toppled the government in a matter of days.

With Iran and Russia in the background, Turkey is now in prime position to play a key role in Syria. Moscow hopes to retain some of its naval and air bases, but the fate of its negotiations with Hayat Tahrir al-Sham is uncertain.

Meanwhile, the United States has maintained a small military presence in Syria in the fight against the Islamic State terrorist group and is allied with Kurdish-led forces that Turkey considers an enemy. And Israel has seized Syrian territory near the Golan Heights as a buffer zone and is conducting extensive airstrikes against what it says are Syrian military and weapons targets.

Syria’s neighbors and European nations – which host millions of Syrian refugees – are watching closely to see if the country can achieve stability or if it will descend back into violent chaos.

Iran

Iran’s powerful network of regional alliances has unraveled, leaving the country vulnerable — and potentially encouraged to build nuclear weapons.

Long considered one of the Middle East’s most influential powers, Iran has emerged severely weakened after a reshuffle over the past 15 months. It has effectively lost much of its once-powerful “axis of resistance,” the network of allies it used to counter the influence of the United States and Israel.

Its closest partner, Hezbollah, is now too weak to pose a serious threat to Israel. And with Mr. With al-Assad ousted from Syria, Iran has lost its influence in a country that provided a critical supply line for weapons and militants.

The previous red lines that kept the region safe from all-out war have been erased: From Israel killed the political leader of Hamas, Ismail Haniyeh, while he was a guest in Tehran, Iran and Israel carried out direct airstrikes on each other.

Exactly where this leaves Tehran is unclear. A weakened Iranian government that feels increasingly vulnerable could be forced to weaponize its decades-old nuclear program. US officials have warned that it may take only a few weeks for Iran to enrich uranium to bomb-grade levels.