Complex sentence: The Imperial Japanese Navy had been planning the "Kongō" class ships from 1909, and was determined that, since the Japanese economy could support relatively few ships, each would be more powerful than its likely competitors.
Split into simpler sentences: The Imperial Japanese Navy had been planning the "Kongō" class ships from 1909. The Japanese navy was determined that, since the Japanese economy could support relatively few ships, each ship would be more powerful than its likely competitors.