The DEFCON system was created during the Cold War, when the threat of nuclear war was at its peak. It's like having a fire alarm in your house - you hope you never need it, but it's there just in case. The highest DEFCON level ever reached was DEFCON 2, which happened during the Cuban Missile Crisis in 1962.
Just like how you might have a "break glass in case of emergency" fire extinguisher, the DEFCON system is a way to respond to potential threats quickly. During the Cuban Missile Crisis, the US and the Soviet Union came close to nuclear war, and the DEFCON level was raised to 2, indicating a high level of alertness. It's like the whole country was holding its breath, waiting to see what would happen next.
