To do otherwise, he said, would have required the United States to be “all in and willing to take over Syria.”
The recent developments in the Syrian conflict send a message to autocratic leaders in the region and elsewhere that force works — and brings few consequences, said Maha Yahya, the director of the Carnegie Middle East Center in Beirut.
The lesson for the victims of that force is that they are on their own.
“Everybody has been watching helplessly as this conflict unfolds,” Ms. Yahya said. “They are watching civilians being massacred mercilessly and all they can do is tweet about it and sign petitions.”
This is the Middle East that President-elect Donald J. Trump will face upon taking office next year, a region where jihadists have erased borders, Russia is ascendant, Iran has extended its reach through powerful militias and American allies are questioning how much they can rely on Washington.
Mr. Trump has articulated no comprehensive policy for the region, other than underlining his support for Israel and suggesting he could work with Russia against the Islamic State, perhaps establishing “safe zones” in Syria — an apparent contradiction since Russian jets have bombed civilian areas.
But the fallout from Aleppo highlights the dynamics that are likely to shape the region throughout his term.
Analysts have begun to add Aleppo to the list of places where humans have failed to stop tragedies committed against other humans, as in Grozny, Rwanda and Srebrenica. The comparisons are not perfect, but can be instructive.
Most estimates put the death toll in the Rwandan genocide much higher than that of the entire Syrian war, although the killing in Rwanda happened much faster, giving foreign powers less time to react.
The siege and bombardment of Aleppo, on the other hand, came after years of conflict in which Mr. Assad’s forces attacked protesters, dropped exploding barrels on rebellious communities and used chemical weapons on their own people.
What is more, because of smartphones and the internet, the Syrian conflict has arguably been better documented than any armed conflict in history. But that has still failed to bring about accountability.
“Aleppo is now the symbol of how far we have retrenched,” said David M. Crane, a veteran international war crimes prosecutor and a professor at the Syracuse University College of Law. “It is part of a worldwide move away from a global village. Countries are turning back into themselves.”
While acknowledging the current weakness of international justice, Professor Crane has been working throughout the Syrian conflict to compile evidence of possible war crimes against different parties in hopes that they will one day be held to account.
“I really do believe that over time we will be able to move forward,” he said. “International justice is not going away.”
By way of example, he mentioned Charles G. Taylor, the former president of Liberia, whom Mr. Crane helped put behind bars in an international trial many years after he had committed his crimes.
The Syrian conflict did not begin as a civil war but as a popular uprising aimed at ousting Mr. Assad. He responded to protests with gunfire, detentions and torture. Many in the opposition took up arms to defend themselves and fight back, drawing support from Gulf countries, Turkey, the Syrian diaspora and the United States. The conflict escalated from there, as Mr. Assad sought help from Russia and Iran.
As the state receded and chaos spread, jihadist movements established themselves, attracting recruits with religious fervor and ample funding, fueling accusations by Mr. Assad that his opponents were terrorists.
Over time, as the space for civil activism narrowed, that claim became increasingly true, giving Western nations another reason not to intervene.
Mr. Obama denounced Mr. Assad as an illegitimate leader but kept American forces out of the battle to oust him. He argued that the United States could not resolve the conflict and that Syria was not a core American interest. Even when Mr. Assad deployed chemical weapons, crossing a “red line,” Mr. Obama did not bomb Syria, angering the opposition and allies like Saudi Arabia, who felt he had further empowered Mr. Assad. Instead, Mr. Obama made a deal with Russia to rid Syria of chemical weapons.
But the war metastasized, spawning new horrors that increasingly affected the United States and its allies. The Islamic State seized territory in Syria and Iraq, declaring a caliphate and inspiring attacks from Bangladesh to San Bernardino, Calif.
And the violence sent waves of refugees into Lebanon, Jordan and Turkey and let loose a flow of migrants whose arrival in Europe has undermined its unity and stability.
All those shocks contributed to the environment in which the siege and battering of eastern Aleppo could take place, changing the course of the conflict.
Mr. Assad’s seizure of Aleppo will leave the opposition with no control in any of Syria’s major cities, possibly signaling its end as a political force that can pressure the government to negotiate.
“The Assad regime has won the strategic war,” said Hassan Hassan, a resident fellow from Syria at the Tahrir Institute for Middle East Policy in Washington. “Psychologically, the opposition is no longer seen as a force that can break Bashar al-Assad.”
Other leaders have paid attention to how he did it.
“The Assad playbook now is that you can crush your people; you can destroy cities; you can attack with chemical weapons; you can enable extremists — and the international community will stand by and not do anything,” Mr. Hassan said. “That is a precedent for dictators who feel threatened by their populations.”
But Mr. Assad’s seizure of Aleppo does not mean the end of the war. Gulf states like Qatar have said they will continue to back the rebels, and many analysts predict that the movement will become a prolonged insurgency.
Mr. Assad’s surprise loss of the ancient city of Palmyra last week to the Islamic State indicates that his fighters are stretched thin.
Also converging in Aleppo is the region’s rising sectarian split. As the rebels have been adopted by Sunni powers such as Saudi Arabia, Mr. Assad has deepened his reliance on Shiite militias who receive support from Iran. Bolstering Mr. Assad’s troops in Aleppo were fighters from Lebanon’s Hezbollah and Shiite militiamen from Iraq and elsewhere who viewed the battle in religious terms.
Many Syrians, including in government-held parts of Aleppo, will be happy when Mr. Assad takes back the whole city because they see him as a symbol of a unified state or because they distrust the rebels for accepting support from foreign powers. Others will just be glad the fighting has stopped.
For some, the war’s greatest casualty has been the ability of Syrians to live together.
Samir Altaqi, a surgeon and former member of the Syrian Parliament who now directs the Orient Research Center in Dubai, said he now avoids images of Aleppo, where he grew up and began his career.
“I don’t bear to look too much at this footage because it would mean a full moral collapse, and I would become too extremist,” he said.
His interactions with younger Syrians who have lived through the war have scared him, he said.
“My impression is that these people have no more distance from death,” said Mr. Altaqi, who is in his mid-60s. “They are sorry to be alive because all their beloved people are dead.”
He recalled his youth in Aleppo decades ago, when his family had Jewish and Christian neighbors and a “mercantile attitude” pervaded the city.
“I remember how we never asked about the religion of our neighbors and friends,” he said, even when a son or daughter brought home a potential mate.
“What will happen to all of this history?” he asked.