Insiders have speculated about a potential falling-out between Donald Trump and Elon Musk for months. It never seemed to happen — until Thursday. The world’s most powerful man and its wealthiest began attacking each other in a spectacle that left Washington breathless. The feud began with Musk’s criticism of Trump’s legislative agenda, which would increase the deficit while eliminating tax incentives that have helped his electric automaker Tesla. Trump responded by threatening to cut government subsidies and contracts for Musk’s companies, and things only escalated from there.