The former Soviet neighbors are surely nervous. Vladimir Putin enjoyed the humiliating US retreat, particularly since he well remembers the lines of Soviet tanks leaving that country in 1989 and the impact they had on Soviet international standing. But instability in Afghanistan threatens Russia, which continues to list the Taliban as a terrorist group, with support for radical Islamic extremism, particularly in the South Caucasus. The governments of Central Asian states Uzbekistan and Tajikistan are right to fear a potential wave of refugees if anti-Taliban resistance continues to grow in Afghanistan’s north, and the presence inside that region of ethnic Uzbeks and Tajiks add more risks of political instability in Central Asia.

Iran is also happy to see the US government look inept, but the Taliban’s return poses trouble there too. NATO-occupied Afghanistan was a crucial source of dollars via cross-border spillover from Western aid. It will now become a source of refugees, drugs, and economic instability.

No country in the neighborhood has changed more over the past 20 years than China, which will now, for the first time, play an active role in trying to keep Afghanistan stable. Beijing needs to ensure that Afghanistan doesn’t become a haven for extremists angered by China’s persecuted Uyghur ethnic minority and a launchpad for terrorist attacks there or elsewhere in China. Beijing also fears a spillover of violence from Afghanistan into Central Asia, where China has invested heavily in President Xi Jinping’s ambitious “Belt and Road Initiative,” infrastructure projects designed to boost China’s economy and international influence.

Even Europe has more at stake than the US in Afghanistan. EU governments fear from Afghanistan a return to the migration and terrorism fears sparked by Syria’s civil war and unrest elsewhere in North Africa.

As Americans pause to reflect on all that’s changed inside their country and for the US role in the world over the past 20 years, understanding the limits of US interests in Afghanistan should play a role in how they think about the next 20 years. The greatest failure since 9/11 comes not from the inability to build democracy in Iraq and Afghanistan or to remake US relations with the Muslim world. It’s in the failure of our leaders—and the American people—to extend the unity that existed after the attack to help the United States build a more perfect union.

Two decades later, the greatest tragedy for the United States is the likelihood that another terrorist attack on US soil would further divide the country. A Gallup poll taken in the days after 9/11 gave George W. Bush a 90 percent approval rating as Americans united in support of their president and the ideal of consensus. It didn’t last, of course, but there was a brief moment when Americans were reminded of all they have in common. A generation later, the question we must answer is whether those days are gone for good.