U.S. lawmakers — both Democrats and Republicans — and top national security officials in the Trump administration offered the same advice publicly and privately, often clashing with Trump’s Twitter stream: the United States remains staunchly committed to its European allies, is furious with the Kremlin about election interference and isn’t contemplating a preemptive strike on North Korea to halt its nuclear program. But Trump himself engaged in a running counterpoint to the message, taking aim on social media at his own national security adviser, H.R. McMaster, because he “forgot” on Saturday to tell the Munich Security Conference that the results...