Richard Grenell has made a career of confounding expectations. That hasn’t changed since becoming U.S. ambassador to Germany last year. On his first day of work, he tweeted a warning shot after President Trump pulled out of the Iran nuclear deal: “U.S. sanctions will target critical sectors of Iran’s economy. German companies doing business in Iran should wind down operations immediately.” The reaction from the buttoned-up diplomatic community was swift. “Never tell the host country what to do, if you want to stay out of trouble,” admonished a former German ambassador to the U.S. But “trouble” is in the eye of the beholder: In just the past eight months, Grenell has convinced blue-chip German companies to pull out of Iran’s volatile market, stopped Chancellor Angela Merkel’s government from sending more than $400 million in cash...