UK law enforcement agents are summoned to the scene of what appears to be the murder of a well-loved schoolteacher from the outskirts of London. It is not thought to be terrorism-related or high profile in any significant way. But it matters enormously to the community, her family, and the many current and former students who adored her. The victim’s ex-husband, John, is the prime suspect. Law enforcement authorities act quickly and get a warrant for John’s stored emails. They serve the warrant on Google, the provider of his Gmail account, but are told: “Sorry, we are prohibited by U.S. law from turning over the content of communications to foreign governments. You need to make your request directly to the U.S. government.” They do, employi...