Maine Sen. Angus King is one of many Kremlin-watchers unnerved by the Russian president’s increasingly mercurial behavior and nuclear saber-rattling.

“Vladimir Putin today is the most dangerous man in the world,” says King, “perhaps the most dangerous man in the history of the world, because he has this blind ambition to reunite the greater Russia at the same time he’s sitting on one of the world’s great nuclear arsenals.”