The largest backward steps taken by navies

What are the greatest backward steps taken by major navies?

Leading examples include the post-Civil War US decline, Britain's loss of conventional carrier aviation, and Ming China's abandonment of its fleet.

The discussion looks for deliberate or structural losses of capability rather than battlefield defeats. The French Jeune Ecole provides the starting comparison, but the post-Civil War US Navy is presented as a more severe collapse in active strength and new construction. Britain's retreat from CATOBAR aviation surrendered a place among the leading carrier powers even if the fiscal reasoning was understandable. Ming China's transition from treasure fleets to almost no navy within a decade is offered as the most dramatic historical example.