Question

Why is Japan called "the great Britain of the east"?

Answer 1

Sea power isn’t a natural attribute of statecraft. Nation states may, of course, decide to develop potent fleets to boost national power, dispute control of maritime spaces and enhance international status—just take the examples of the ‘treasure fleets’ of the Ming dynasty or the High Seas Fleet of Imperial Germany. But generally speaking, a government’s commitment to continuously funding, expanding and modernising the naval core of its military capabilities is tied to how much the country’s a strategic imperative.

Answer 2

Both were involved in a civil war between two Royal Families in which one side flew red flag and the other white flag.

Namely The wars of the roses and the Gempei War..