What happened
The Soviet Union blocked ground access to West Berlin in response to western moves in occupied Germany, prompting the United States and its allies to supply the city by air.
Why it matters
Berlin became the first sustained Cold War flashpoint. The crisis hardened the division of Europe, made Germany central to bloc politics, and demonstrated that direct confrontation could stop short of general war while still carrying major strategic risk.
Notes
This is the main early-crisis anchor for divided Europe.