President Donald Trump took his bellicose economic agenda abroad last week, blasting Germany for its “very bad” trade surplus—or “evil” as one German newspaper translated it.

Though German Chancellor Angela Merkel did not seem to care for the messenger, she should nonetheless hear the message. While Mr. Trump gets a lot wrong about trade, on this particular point he’s right. Germany’s current account surplus, which combines trade and investment income, is now the world’s largest. Along with China’s, it is a dangerous imbalance...