Military experts say a new generation of nuclear weapons has raised the risk that Mr. Putin might introduce less destructive atomic arms into the battlefields in and around Ukraine.
When the Soviet Union collapsed, Ukraine turned over thousands of atomic weapons in exchange for security guarantees from Russia, the United States and other countries.
A new study offers insights into how the United States reacted to Tsar Bomba, a planet shaker that made the deadly Hiroshima blast look tame.
Experts report that similar technologies were developed by Russia and the United States starting more than a half century ago.
The military’s satellites excel at spotting new blazes, but for decades they have been mostly off limits to civilian firefighters.