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Credit for summary Attack on Titan Season 4 Wikipedia.
During Marley's expansionist phase, it is engaged in a four-year war against the Mid-East Allied Forces and is attempting to conquer Paradis Island. Eldian soldiers are being used as cannon fodder to attack Fort Slava, including Falco, one of the young Eldian warrior-candidate soldiers, along with his older brother Colt.
Colt proposes to General Magath that they use the power of the Warrior Titans, but he refuses due to the the Allied Forces having an armored train, along with canons capable of destroying Titans.
Gabi, a promising candidate to be the Armored Titan and Reiner's younger cousin, manages to approach the train while appearing unarmed and disables it with hand grenades. This allows Pieck's Cart Titan, Galliard's Jaw Titan and Reiner's Armored Titan to attack the fort.
From an airship above Fort Slava, Eldian humans are transformed into Pure Titans by Zeke and are dropped down rashly, causing destruction and mayhem. Zeke then transforms into the Beast Titan and throws ammunition towards the ships of the Allied Forces, sinking the fleet.
The Marleyans capture the fort and win the war with a heavy sacrifice by the Eldians, however the existence of weapons capable of injuring even the Warrior Titans causes Marley to realize that they need the power of the Founding Titan more than ever.