The Fall of Constantinople
For over a thousand years the walls of Constantinople had made it nearly impregnable — until the Ottomans battered them down with enormous gunpowder cannon, extinguishing the Byzantine Empire, the last living fragment of Rome. The blast marked the moment gunpowder rendered the old world of castle walls obsolete. It also cut Europe off from the overland routes east, spurring it to seek a sea path to Asia — the search that would soon send ships across the Atlantic.
A weapon or tactic upends the prevailing balance of power and renders an old defensive or offensive order obsolete.