My personal fanon for Crusaders 10K, is the Sirens are each controlling one side behind the scenes, between the three of them secretly making no side truly loses or truly wins, only continue to endlessly conflict with each other adding more to feed them, and such a full scale war on Equestria WOULD power them up to be basically goddesses.
"The Battle of Cloudsdale was an essential battle in the War of Griffonian Succession. General Iron-Tail, the griffon who won victory in the Siege of Ponyville, sought to conquer Cloudsdale using the same weapons used to conquer the eastern half of the commonwealth; gunpowder. Professor Himrick Wulf, a wolfish philosopher from the northwestern kingdom of Ferax, theorized that if powder from raw magic crystals were brought to spark, it would produce the same results as the common griffon musket. Princess Celestia wasted no time to order her highest war wizards to test this theory and, to the wonder of all of Equestria, it worked. The powder was soon weaponized in a similar form as a hand-cannon and was quickly rushed off to Cloudsdale to defend the floating city.
"The griffon army arrived on the 6th of April, 1006. With 10000 infantry, wielding pikes, swords, and muskets, 4000 heavy pegasi cavalry, and around 30 cannons stationed around the city, the ponies figured that fighting them would be suicide since their garrison held no more than 6000 infantry and 10 cannons. However, both armies were shocked to see Princess Celestia and the Rainbow Warriors, her personal legion of sullied soldiers, charging through the invading infantry and cavalry. Wielding the new magical muskets, now dubbed the Smoke-Pole, the griffons were quickly overwhelmed and Iron-Tail was killed by a stray magical bullet. By the end of the day, the griffons were either dead, captured, or retreating; victory was awarded to the ponies and Celestia set her sights on reclaiming her home: Canterlot."
~ Excerpt from "A History of Equestria: From Ancient Alicorns to the Modern Day" Written by Ink Rose of the Mage's Collage of Canterlot