Luckily for Beijing, the Afghan war — along with Iraq and other American misadventures in the Middle East — distracted Washington for two decades. While China was building roads and ports from Beijing to Trieste, Italy, fueling its economy and expanding its geopolitical influence, the United States was pouring money into its war on terrorism. While Beijing was building thousands of acres of military bases in the South China Sea and enhancing its precision-strike capabilities, the U.S. military was fighting an insurgency and dismantling improvised explosive devices.

In many ways, it was just dumb luck that Mr. Xi and his predecessors, thanks in part to the war in Afghanistan, could build national power, undermine international norms, co-opt international organizations and extend their territorial control all without the United States thwarting their plans in any meaningful way.

But the end of the war in Afghanistan could bring these good times — which the Communist Party calls the “period of important strategic opportunities” — to an abrupt end. Sure, over the past 10 years American presidents tried to get back into the Asia game even as the war continued. Barack Obama asserted we would pivot to Asia back in 2011. Donald Trump’s national security team made great power competition with China its top priority.