Marley is a nation located beyond the Walls and across the sea from Paradis Island. Marley was once conquered by Eldia in ancient times and subjugated by them for countless generations, until the Great Titan War when the Marleyans rose up and subjugated Eldia's territory, except for Paradis Island.
In modern times, Marley sustained its global power using the power of the Titans of its Eldian populations that they gathered inside internment zones to live as second-class citizens, while attempting to seize Paradis Island to capture the Founding Titan and the island's natural resources. By the time of the Rumbling, the nation of Marley managed to conquer huge swathes of land and subjugate a numerous amount of countries and peoples in its colonial empire.
Marley served as the main antagonist to the Eldians of Paradis Island and were the ones that filled Paradis Island with Titans. In the year 854, Marley officially declared war on Paradis Island, to which they lost when Eren Jaeger activated the Rumbling, leading to the extermination of countless Marleyans and the trampling of the vast majority of the continental mainland.
History: Marley was once a powerful nation in ancient times; however, this changed over 2,000 years ago when a slave of the Eldians named Ymir gained the power of the Titans. King Fritz used her abilities to defeat Marley and force their surrender. Ymir became Fritz's concubine and conceived three daughters with him.
Story: After Marley's failed operation, the Mid-East Union, a coalition of various nations, took advantage of the defeat and declared war on the nation, starting a four-year long war. A significant portion of the war took place on the sea, with Marley gaining the upper hand over the course of the four years, at the cost of half of its navy. During the last battle of the war, the Marleyan forces, with the help of its infamous Warrior Unit, was able to conquer Fort Slava and destroy the Mid-Eastern fleet.
Trivia: The national emblem of Marley appears to be a seashell motif and is found on several structures located in Liberio as well as official military paperwork. The seashell motif is also found engraved in the ancient Marleyan flag, on the modern navy's hats, on the Tybur family personnal guards' spiked helmet, and in the current official currency of the country. Marley draws on Latin and Italian inspiration for much of its naming and culture.
Source: Marley (Anime), available under CC BY-SA 3.0.