The 1915 Panama Colombia Exposition was supposed to be San Diego's quinceanera, where it would set itself as the nation's premier gateway to the pacific. Instead, the lack of a direct railroad connection to the east coast ensured LA would gain that title. Five years later the city leaders sold virtually the entire bay to the navy, which set SD on the stable course it lies on today instead of the limitless growth they'd previously envisioned.
But hey, at least we got some pretty cool architecture out of the deal.