Despite repeated messages of support by Trump, Waltz has lost sway with the president and the backing of senior aides within the White House, officials said, just as the administration struggles to broker peace deals and faces the threat of further war in the Middle East. For Trump, Waltz’s biggest sin wasn’t starting a Signal chat to coordinate strikes on the Houthis in Yemen, or even posting Israel-provided intelligence onto an unclassified network, it was having the Atlantic magazine’s editor in chief Jeffrey Goldberg’s number in his phone and inadvertently adding him to the conversation. Trump’s anger spilled over into...