Since the atomic bombings of Hiroshima and Nagasaki, more than 2,000 nuclear weapons have been tested for military purposes, but only the United States, Russia (now including Belarus), China, France, India, Israel, and Pakistan are known to possess them.
Even a limited nuclear attack could cause an enormous catastrophe. A comparatively small number of high-yield weapons would destroy most of the major cities in North America and Eurasia, killing millions. In addition, there would be millions of people outside the area of total destruction who would suffer from severe burns, blindness, and internal injuries caused by collapsed buildings and flying debris. Emergency services, power supplies, water resources, and communications would be devastated. Food, clothing, and shelter would be scarce.
An attack on hardened nuclear targets, such as intercontinental missile silos or submarine support bases, might be more contained. But such attacks would still kill millions of people from blast and local radiation effects. Air bursts dig a large crater and pulverize everything within several miles; ground bursts, which are typically used against hardened military targets, cause similar damage but the destruction extends much farther because the radius of destruction is proportional to the explosive yield.
In the event of an actual attack, listen for official information and follow instructions. Take cover as far below ground as possible or in the center of a multistory building. If possible, avoid areas where radioactive material has been deposited and stay inside until instructed to evacuate. During periods of heightened threat, it is a good idea to increase your disaster supply kit so that you have enough supplies to last for up to two weeks.