Dark fears that have been largely dormant for over three decades are resurfacing because of Russia's six-week-old invasion of Ukraine. The nightmare question is whether Russian leader Vladimir Putin might use nuclear missiles – a mostly unthinkable nightmare since 1991, which brought the end of a "Cold War" between the United States and the Soviet Union (as Russia's larger predecessor was called). Now Putin has put his country's nuclear weapons on a higher alert status, meaning they're closer to launch readiness. His foreign minister, Sergey Lavrov, warned two weeks ago that the threat of a nuclear war "cannot be underestimated. . . . The danger is serious."

In Washington, President Biden called Russia's actions and talk "irresponsible," and said: "No one should be making idle comments about the use of nuclear weapons or the possibility of the need to use them." The U.S. ambassador to the United Nations said recently: "President Putin is continuing to escalate this war in a manner that is totally unacceptable. And we have to continue to condemn his actions in the most strong, strongest possible way."

Russia, like the U.S. and NATO, has thousands of nuclear warheads as a deterrent to an attack. A doctrine of "mutually assured destruction" — that no state will start a nuclear war because swift retaliation would jeopardize its own fate — has kept those ultimate weapons from being launched. But some experts say the fear of a major nation's nuclear missile launch now pushes smaller states, such as South Korea, to consider nuclear weapons for protection. Ukrainian President Volodymyr Zelensky says his country shouldn't have abandoned nuclear arms inherited from the Soviet Union.

Pentagon leader says: "Any time a senior leader of a nation-state starts rattling a nuclear saber, then everyone takes it seriously." – Gen. Mark Milley, chairman of the Joint Chiefs of Staff

British expert says: "It's outside the realm of possibility right now that there's going to be a nuclear war or World War III that really spills over that far beyond Ukraine's borders." -- Samuel Ramani, analyst at the Royal United Services Institute, a London research center

U.S. historian says: "I sense a period ending. I am now deeply afraid that Mr. Putin's recklessness may cause the years between the Cold War and the Covid-19 pandemic to seem a halcyon period [peaceful time] to future historians, compared with what came after." -- Mary Elise Sarotte of Johns Hopkins University in Baltimore