This is a very simplified political map of Japan showing various prefectures in the northern end of Honshu, the largest of Japan's four main islands. Our area of interest today is the region marked by the bright red pin.
If you zoom into this area, you will notice a very peculiar cartographic oddity. The borders of Fukushima, Niigata and Yamagata doesn’t meet at a tri-point as the zoomed-out map seems to suggest. Instead, you will see a very thin strip of land attached to Fukushima snaking into Niigata. This is called a salient. The Fukushima Prefecture salient —famously called the umbilical cord—extends about 8 km starting from the summit of Mount Mikuni, following the ridge of Mount Kengamine, passing through the summit of Mount Iide, and ending on the summit of Mount Onishi. At its narrowest, it is only about 35 inches across (90 centimetres).
Why does this strange piece of border exist?
Before Japan’s modernization in the 19th century, the area now known as Fukushima Prefecture was divided into several han, or feudal domains, ruled by various daimyō under the Tokugawa shogunate. The powerful Aizu Domain ruled the mountainous west; Iwaki and Sōma Domains governed the coastal east; and other smaller domains and shogunal territories filled the spaces between. The borders between them often followed old trade routes and mountain ridges, not any single administrative logic.
When the Meiji Restoration of 1868 swept away the feudal order, the new government replaced the domains with a modern prefectural system. At first, there were hundreds of small prefectures, each roughly corresponding to an old domain. Over the next few years, these were consolidated into the larger, more manageable units seen today. In this process, the Aizu lands were merged with the coastal regions to form Fukushima Prefecture — a political decision meant partly to diminish the lingering power of the Aizu samurai, who had fought fiercely against the imperial forces during the Boshin War.
By the mid-1870s, the map of northeastern Japan had begun to resemble its modern form, but the details of prefectural boundaries, especially in the mountains, remained fluid. At the heart of these neglected highlands stood Mount Iide, a soaring peak on the border between the two prefectures. On its slopes lay Iide Shrine, a sacred site venerated by locals for centuries. The mountain and the shrine had long been associated with the former Aizu Domain, and hence with what became Fukushima.
In 1886, the Fukushima Prefectural Assembly proposed moving the prefectural capital from Fukushima City to Kōriyama, partly because access was difficult from some areas. As part of balancing and administrative convenience, the national government approved transferring Higashikanbara District, which included the eastern approaches to Mount Iide, from Fukushima to Niigata Prefecture. The redrawn border placed Mount Iide’s summit and the upper shrine under Niigata’s jurisdiction.
This change did not sit well with the residents of nearby villages in Fukushima. For them, Mount Iide and its shrine were not mere landmarks but part of their cultural and spiritual identity. Losing them felt like losing a piece of their heritage. Local officials and villagers petitioned the government, arguing that historical records and old domain maps proved the mountain had always belonged to Aizu.
For more than twenty years, the issue simmered. Finally, in the early 1900s, the Home Ministry ordered a field survey to settle the matter. The surveyors confirmed that the disputed shrine and the ridge leading up to it had deep historical and cultural ties to Fukushima. In 1907, a compromise was reached: while the rest of Higashikanbara District remained in Niigata, the border was redrawn to follow the ridge of the Iide Mountains, returning the summit and the shrine to Fukushima. This resulted in this narrow strip of land.
The approach to Mount Lide along the ridge belongs to Fukushima Prefecture. The left slope belongs to Niigata Prefecture, the right slope belongs to Yamagata Prefecture. Credit: Wikimedia Commons
References:
# Iidesan, Fukushima’s Odd Border: Gods, Mountains, Lines in the Sand, Unseen Japan
# Japan's most mysterious prefectural border! Fukushima Prefecture cuts between Niigata and Yamagata prefectures!?, Gendai.media
# What is the true identity of the "navel cord" in Fukushima?, Nikkei
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