Storm Wars

The Storm Wars was a massive war that stretched across The Mysterious South, Equestria, Seaquestria and the retconned continent of Land of the Hippogriffs (retconned to be Mount Aris. The war was essentially a series of conquests performed by the Storm King with many one-sided battles. It was unclear when exactly the event started, but it ended after the Liberation of Canterlot, though its effects were felt for about a year or two onward at least.

Abyssinia and Mount Aris
By the time the Storm King sacked Abyssinia, he was known as the "destroyer of a dozen lands", meaning his exploits were already well known and dreaded. According the Abyssinian king, he had already took over their lands, but he conquered them again simply because he could. Sometime around this point, though unknown if before or prior, he had all but completely destroyed the kingdom of Mount Aris, causing the immortal kingdom of the Hippogriffs to flee into the sea, founding the land of Seaquestria in their wake.

Tempest Shadow
In the wake of the sacking of Abyssinia, Tempest Shadow came across the wreckage of the S.S. Meow, a vessel used by Chummer, one of the survivors of the Abyssinian assault who had fled to a pirate ship. In the wreckage, she came across the Misfourane Malachite, an object that intertwined her destiny with that of the Storm King. After being recruited into his army, she claimed to have toppled entire kingdoms while in his service, implying the Storm Wars scope was even bigger than we'd previously imagine.

Sacking of Canterlot
The Storm Kingdom found themselves in Canterlot during the cities Friendship Festival celebration. They attacked fast and harm with no mercy, petrifying Princess Celestia, Princess Cadance, nearly doing so with, blowing up the Canterlot bridge and enslaving the population all within minutes.