Foreign policy rarely plays a significant role in a US presidential election. Domestic issues, the economy, money in the pocket, jobs, immigration, these are what voters are most concerned about. But this time, it could be different.

The first TV debate between President Biden and Donald Trump seemed focused on one thing: is Biden the man to trust to lead the western alliance for another four-year term? Or, as Trump insinuated, is he so weakened and fragile that none of the adversarial leaders in the world have any respect for him, let alone fear him?

For America’s allies, Biden’s lacklustre performance will...