The Royal Islands.
The home of one of the greatest seafaring nations the world has ever seen. For over half a century it was the ships of the Royal Navy that dominated the seas and ruled the world of maritime commerce. Challenged only by the Iris Orthodoxy, the Iberian Kingdom and the traders of the Ijesselmeer. All of them humbled and defeated by the might of the royal navy. This hegemony of absolute domination of the seas by the Royal Navy lasted for almost 300 years.
But by now it was challenged by others, new nations that claimed their right to sail the seas. The reformed Sakura Empire made an alliance with the Royal Navy to build up its own navy and to make it the dominant seapower in the pacific ocean. The Northern Palairment, though always a maritime power to be reckoned with in the baltic and black sea, now started to make its claims in the pacific and north atlantic known. Then there was the Eagle Union. The country that used to be a colony of the Royal Islands about 250 years ago, now they claimed their right to the caribbean, Atlantic and Pacific ocean. Though isolationist their industrial power allowed them to potentially keep up with the fleet programm of the Royal Navy. And then there was Ironblood. The nation that used to be many small ones. It had a maritime tradition along the north sea coast. Every small state and city state by itself was not strong enough to ever challenge the Royal Navy. But the Royal Islands remembered the united power of the Hanseatic League well. Now that these lands were united and their potential combined, they were able to challenge and match the royal navy. The ensuing naval arms race between these emergent powers and the Royal Islands lead to the point, that so many nations build up their navies to what they were, when the great war erupted.
It was about who was going to have dominance over the seas and continents. Would the Royal Islands be able to keep their place as the dominant power, or would one of the new or old friends and foes replace them? Due to their tradition and experience in shipbuilding, the Royal Navy was able to outbuild their enemies. Especially the Ironblood and the threat in mediterranean sea, the Sardegna Empire. The Sakura Empire betrayed the decade-long naval alliance and started to fight the Royal Navy and Eagle Union. The latter now joining the war, was able to use it's industrial might to outbuild everyone. The Royal Navy was able to hold its ground against its enemies, but it's ally the Eagle Union surpassed it in size and power. The former hegemon of the seas was now a viceroy when the Sirens appeared and wreaked havoc.
Since they appeared the great naval powers agreed to a ceasefire to use their power to fight the new collective enemy. The royal navy was now able to replenish its forces and secure its place as the second strongest power on the sea, even though this situation was one they rather not acknowledge.
It was the Royal Navies' idea to organise a meeting of all navies with interests in the Atlantic ocean and its connected waters to agree on terms of how the navies should coordinate to defend the ocean against Siren attacks. But it was mainly done to reduce the risk of conflicts between the navies, technically still at war with each other. The fact that the come to live Shipgirls of each navy tended to have dislikes for their traditional enemies, did not help that matter.