Connecting through a country doesn't always mean you can skip its entry rules.
Common situations
- Staying airside on one ticket: many countries don't require a visa, but some do for certain nationalities.
- Connections in the U.S.: there is no airside transit; you must enter the U.S., so you need an ESTA or visa even to change planes.
- Changing airports (for example two London airports) means entering the country.
- Self-transfer where you collect and re-check bags usually means passing immigration.
How to check
Use the airline's travel-document checker (most use IATA's Timatic database) and the official government site of the connecting country.