Kantai Kessen was not simply Mahan plus the Jeune École

Was Kantai Kessen essentially Mahan and the Jeune École with submarines and aircraft carriers added?

Only partly. Foreign theories influenced Japanese officers, but Japan adapted them through its own strategic tradition and institutional experience.

Kantai Kessen shared features with Mahanian battle doctrine and the Jeune École's emphasis on smaller attacking forces. Japanese officers read foreign writers and served abroad, so Western ideas entered their professional debate. Treating the doctrine as a simple import, however, erases Japan's own history of warfare and strategic thought. Foreign examples passed through Japanese institutions and assumptions instead of being copied unchanged. The cruisers' combination of scouting, torpedo attack, and battle-line support expressed that distinct synthesis in material form.