The Battle of Eclipse · 2:57pm May 12th, 2018
(Canon to the Advancedverse)
Battle of Eclipse (-632 P.D.E.): The first and only attempt by the Crystal Empire (expanding south) to seize territory from the Republic of Equestria (expanding north) began and ended with the Battle of Eclipse, in which the hitherto-undefeatable crystal unicorn ability to lase (which, when grouped into phalanxes, permitted them to reduce fortifications and eliminate close formations in a matter of seconds) was countered by the emergency measure - proposed and implemented by the collegium caelis1 - of moving the moon and the sun in syzygy, producing an eclipse lasting three days to take away the ambient light upon which this ability depended.
The peace talks that followed were short, and defined the solution to the problem of the irresistible force meeting the immovable object. To wit, don't.
1. The Republic's magistrates in charge of moving the heavens. Originally composed of a changing number of unicorns, by the time of the Battle of Eclipse, Celestia and Luna were, effectively, the entire permanent membership of the collegium.
Had to look back over the timeline to get context for the state of the Empire at the time, but this is an excellent bit of world building. Solar-powered strategies don't work well when the opposing nation has the only celestial mechanics on the planet.
I bet that was a short-lived rebellion. Sorry; I could not resist that pun.
The Crystal Palace can only contain so much sunlight stored bouncing around its structure before it starts to soften. After that, its all over but the signing?
So Equestria was a republic for a while?
4859229
In this continuity, yes - the Republic of Equestria fills in the timeline between the Founders (Platinum, Hurricane, and Puddinghead) and the Discordant Era. While the Princesses were around earlier, they weren't the Princesses until after defeating Discord, setting the world to rights, and generally demonstrating impressive leadership qualities.
(There was not all that much enthusiasm among ponies at the time for restoring the Republic, which after all had failed to prevent the rise of Discord. Not that there was much it could have done against the chaos god, but technical fairness was less on everypony's mind than, say, "Thank you, thank you, thank you so much for making time flow in only one direction again and turning gravity back on!")
4859357
Ok, got it.