Complex sentence: After World War I, the bulk of the High Seas Fleet was interned in Scapa Flow, but the "Nassau"-class ships, the oldest German dreadnoughts, were initially permitted to remain in German ports.
Split into simpler sentences: After World War I, the bulk of the High Seas Fleet was interned in Scapa Flow. As they were the oldest German dreadnoughts, the "Nassau"-class ships were for the time permitted to remain in German ports.