Written by an award-winning historian of science and technology, Planet in Peril describes the top four mega-dangers facing humankind - climate change, nukes, pandemics, and artificial intelligence. It outlines the solutions that have been tried, and analyzes why they have thus far fallen short. These four existential dangers present a special kind of challenge that urgently requires planet-level responses, yet today's international institutions have so far failed to meet this need. The book lays out a realistic pathway for gradually modifying the United Nations over the coming century so that it can become more effective at coordinating global solutions to humanity's problems. Neither optimistic nor pessimistic, but pragmatic and constructive, the book explores how to move past ideological polarization and global political fragmentation. Unafraid to take intellectual risks, Planet in Peril sketches a plausible roadmap toward a safer, more democratic future for us all.