With limited strategic depth at home, Japan could not safely wait for an enemy to reach its islands in this analysis. Much of the country's vital infrastructure was exposed to attack by a powerful fleet once it came within range. The sea therefore had to serve as defensive depth, despite the immense surveillance problem created by the Pacific. Before satellites, a defensive naval war depended on finding an approaching enemy over a very large maritime space and acting before it reached Japanese industry and population centers.
Notable Point · 37:37-40:25
The sea as Japan's defensive depth
Japan's geography, fragile economic fundamentals, and exposure to major naval powers made early detection and seaward defence central concerns.