As Cabinet members snarl at representatives and senators, and social media fills with semiliterate trolling and insults by public officials, we need to remember that rhetoric—the art of persuasive speech—still matters. Marco Rubio, the U.S. secretary of state, gave an excellent demonstration of that fact at this year’s Munich Security Conference. Any contemporary speech carries the burden of multiple audiences. In this administration, the first and most important is the volatile and tempestuous president. But at Munich, there are other audiences as well: those in the room who represent not only a European but a global national-security elite; European and other politicians outside the room who care chiefly about domestic politics but are aware of international politics; real and potential enemies; and an American audience taking the measure of its country’s leaders. Delivering speeches at Munich is a perilous business. In 2018, Lieutenant General H. R. McMaster gave a podium-thumping address about nuclear proliferation and jihadists. A month later he was sacked; he’d received a tweeted dressing-down in midair on the way back for having …