In the battle against the coronavirus, few places seemed as confident of victory as China. In recent months, the government has contended with sporadic outbreaks in various provinces, but stamped them out swiftly by mobilizing thousands of people to test and trace infections, as well as locking down communities. That model is now looking increasingly fragile. China is facing its biggest challenge since the virus first erupted in the Chinese city of Wuhan last year: the highly transmissible Delta variant that is rapidly spreading throughout the country. Chinese officials have acknowledged that...