Most airline passengers interact with an alliance before they realise one exists.
The lounge access waiting at the other end of a long-haul flight. The frequent-flyer miles earned on a carrier they have never flown before. The ability to check baggage in London and collect it in Sydney despite travelling on multiple airlines with different owners, different liveries and different national identities.
Today these experiences feel routine. In the late 1990s they represented one of the most ambitious experiments the aviation industry had ever attempted.