Cold War historian Mary Elise Sarotte says a new, more dangerous form of that 20th century conflict may descend upon Europe because of Russia's unprovoked invasion of Ukraine. The nuclear weapons are still around, although fewer in number, but gone are the climate of detente, mutual trust, and most of the major arms control treaties that marked the end of the Reagan years and the early 1990s. The author of "Not One Inch: America, Russia, and the Making of Post-Cold War Stalemate," Sarotte argues the way NATO expanded to the east helped ruin U.S.-Russia relations, but it is far from the only explanation for a war launched by Russian revanchists.