Afghanistan was a battleground for the United States to show its power to China and Russia
Afghanistan was a battleground for the United States to show its power to China and Russia but Britain, along with the United States, lost the war. And it sent a message to China and Russia that they are also weak against people who do not have weapons like the United States or Europe. So is there anything to be gained from joint US-British military exercises? In case if the Taliban retake Afghanistan?
The lesson to be learned from Afghanistan is that people don't believe in your idea, they won't be fighting for it. It took 20 years to train the Afghan Army and it got destroyed in 2 weeks. Most of the soldiers didn't even fight and just went home. Most of the fighting was done by the special forces but they were outnumbered and had no air support. Most of them got wiped out. So all USA did in the end was giving better weapons to the Taliban.