America’s global campaign to prevent Huawei Technologies from building the next generation of superfast internet has faltered on the continent where Washington finds some of its closest allies: Europe.

For months until the coronavirus pandemic, America’s top officials crisscrossed the continent, arguing that the Chinese company is an espionage threat, subservient to its authoritarian government. For Europe to transmit data on networks using Huawei 5G equipment would offer Beijing an easy way to spy on the sensitive information...