The Russian leadership may be afraid the event will showcase its vulnerability.
Archived version: https://archive.is/newest/https://euromaidanpress.com/2025/07/26/russia-cancels-main-naval-parade-after-losing-33-of-black-sea-fleet-in-ukrainian-drone-strikes/
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Had never thought of that! Plus, imagine the humiliation if Ukraine got even a single drone hit on that parade.