Warfare

Warfare is a consistently appearing, if somewhat unspoken, aspect of the mythology of My Little Pony that often appears in the backstory of various places, rarely ever the focus. War is brutal and often ends up either destroying entire realms, or severely damaging them and halting their civilization for some time.

Friendship is Magic
The earliest known conflict between two political factions was the Cat Revolt in Anugypt when a group of Cat freedom fights lead by Baast overthrew the government of Anugypt in one swift battle, presmably allowing cats to have equal rights with and Jackals. Unlike some future wars, this conflict was relatively peaceful, resulting in little known destruction.

The next oldest known is likely the Zephyr War which was fought by early and Griffons. Like the former war, this war appeared to be somewhat peaceful as it was resolved very quickly when a group of Hippogriffs came in and mediated the two sides, resulting in the two armies to come together and form their own nation, the Eastern Equestrian Kingdom.

Presumably over or nearly 1000 years later, it was hinted that Equestria engaged in war with either itself or another power, as the museum Colonial Whinniesburg displayed cannons as exhibits of an ancient battle with several workers reenacting soldiers from that period.

Perhaps the longest and most notorious war in Equestria was the Changeling War, a long campaign lead by in her attempts to conquer Equestria. Her first attempts lead into the Canterlot Wedding event, a large scale battle in which her Changeling army invaded the city of Canterlot after she had disguised herself as and nearly married. The battle was ended when the true Cadance defeated Chrysalis, but the war waged on as Chrysalis conquered Wuvy-Dovey Smoochy Land and built her a new kingdom. From there, she regained her strength and numbers and kidnapped the Cutie Mark Crusaders so she could lure in so she could defeat her. This of course didn't end as Chrysalis pleased and Twilight defeated her and later turned her castle into a prison.

The most recent war was the Storm War, a long campaign in which the Storm King conquered a dozen lands and destroyed a dozen others; the only known of these lands were Abyssinia and Mount Aris which he laid siege upon. The war was ended when he stepped foot in Equestria, though he managed to take Canterlot, Twilight Sparkle and the others eventually managed to stop him with the aid of Tempest Shadow where he was turned to stone and them shattered. Despite the war having a fairly decisive end, the effects of the battle was felt long afterwards. According to some sources, the Storm King may have survived his fall and turned into a being of pure energy. Somehow the Storm Kings magic affected the human world, turning a jungle island into a magical landscape. And most recently, his daughter Swift Foot infiltrated Equestria.

Known wars

 * Cat rebellion
 * Zephyr War
 * Storm War