Control of this island chain was important to the japanese because it gave them a line of defenses far to the east

Explanation:

The mountainous islands of the archipelago form an arc from the east Asian coasts. The national territory includes the small Nanpō Islands, including Iwo Jima Island, approximately 1100 kilometers from the main islands. The peculiarity of Japan being an archipelago means that no point in Japan is more than 150 kilometers from the sea.

The five main islands are separated by narrow channels and three of them (Honshu, Shikoku, and Kyūshū) by the Seto Inland Sea. At the southern tip are the Ryukyu Islands 970 kilometers south of the third largest island, Kyūshū.3

The closest point to the Asian continent is the Korean peninsula at a distance of approximately 200 kilometers. It was always connected to the mainland through sea trade routes: in the north with Siberia, in the west from the Tsushima islands to the Korean peninsula and in the south with the ports of southern China.

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