Aleppo is falling, Aleppo has fallen. Some are already calling it this generation's Sarajevo. Its Rwanda. Its Guernica.

Syrian forces loyal to President Bashar al-Assad are now in control of almost all the city, and will likely take over the rest very soon. On Tuesday, Turkey and Russia brokered a deal to allow rebel fighters, who flooded into the city in 2012 in a push to topple Assad's government, and civilians, to leave the besieged city, but the ceasefire does not seem to be holding. The Syrian Civil Defense group said on Twitter entire streets and buildings were "full with dead bodies." They said they could hear children calling for help but couldn't reach them due to continuous bombing. On Tuesday, François Delattre, France's U.N. ambassador, said, "The worst humanitarian tragedy of the 21st century is unfolding before our eyes."

Over the last six years, the war in Syria has claimed the lives of an estimated 400,000 people.

The fall of Aleppo is a humanitarian and cultural disaster, and it happened under the watch of the world, and under its moral responsibility.

In Syria, there haven't been any good options for a long time. But the international community, led full force by the United States, has picked the worst option at every turn.

A "moderate" Syrian opposition was always more hope than reality, but now it is a complete mirage. Key personnel have been systematically eliminated by Russian and Iranian secret service as foot soldiers were driven into the arms of Islamists.

The deterioration of Syria led to the rise of the Islamic State, to refugee waves that are destabilizing Europe, to a further setback to the West as Iran and Russia's spheres of influence widen, and, of course, to humanitarian disaster on a large scale.

The Assad regime — supported by Iran, and its proxy Hezbollah, as well as Putin's Russia, which has a naval base in the country which is precious to its interests — has perhaps never appeared closer to winning this civil war. When rebels tried to seize Aleppo in 2012, it looked like a decisive bid for power: They would be taking over the commercial capital and building an alternative capital to Damascus.

It didn't pan out that way.

With an axis of control stretching from Aleppo in the north to the Mediterranean coast to Damascus in the South, the Assad regime will now decisively control the country's economic and demographic backbone. ISIS is on its last legs. Things don't look too good for the Kurds, sandwiched between Turkey and Assad. While the war will undoubtedly burn for a long time, and rebels have patrons of their own (although the U.S., under Trump, is likely to tire of arming them), it seems all but certain the Assad regime is going to retain its hold on power.

This is a major humiliation for the United States, which backed the rebels, and a major victory for its geopolitical antagonists, Russia and Iran, which back the Assad regime. In other words, Russia defeated the U.S. This configuration of a newly emboldened and victorious Russia and a weak America will have consequences in the Middle East and beyond. Indeed, America's already-anxious NATO partners are watching.

With the rebellion defeated on the battlefield, it will now turn to the next tactic of choice within die-hard rebel groups who have nothing to lose: terrorism. Between ISIS and al-Nusra, a rebel force aligned with al Qaeda, the Syrian civil war has been churning out battle-hardened Islamist terrorists. The collapse of Aleppo, followed by the military defeat of ISIS, could spawn a never-ending wave of terrorism not only in Syria, but also all across the world.

We have a humanitarian disaster on our hands. There will almost certainly be tens — maybe hundreds — of thousands of refugees. What will happen to them? Will they come to Europe and further destabilize it? Or destabilize Turkey? Or will they stay in Syria and starve?

Those are just some of the many questions that remain unanswered. In the meantime, the world has to realize that Assad is probably here to stay, at least for the next few years. The Assad regime was once a bad option, then it became the least-bad option. Now, tragically, it is the only option.