The United States and Russia are dangerously close to stumbling into a war over Ukraine that could go nuclear and kill hundreds of millions of people in a single day, a Nobel laureate who is one of the world’s leading experts on the dangers of nuclear weapons warned in Washington this week. “It’s an incredibly dangerous situation. …If there’s a nuclear war tonight, that’s the Northern Hemisphere (of the entire world) gone, Dr. Helen Caldicott told a National Press Club Newsmakers news conference on Wednesday. She was speaking on the topic: “Ukraine: Is Nuclear Conflict Likely?” Caldicott is an Australian...