When President-elect Donald Trump officially becomes the president of the United States in January, he will take complete control of America’s nuclear arsenal. Should he decide to start a nuclear war, there are no legal safeguards to stop him. Instead, a much less tangible web of norms, taboos, and fears has reined in US presidents since World War II. He wouldn’t take nuking ISIS, or even Europe, off the table. But he’s also characterized himself as “highly, highly, highly, highly unlikely” to ever use nuclear weapons.