The biggest geopolitical error EU has made is to entrust its defense to USA.
The biggest socioeconomic error EU has made was to entrust its technology and data processing to proprietary USA corporations.
The biggest political strategy failure in EU was to naively believe it could rely entirely on its own to achieve the noble social and environmental goals it had set for itself.
The largest betrayal EU has ever made is to spy on its citizens with the excuse of CSAM