Two Kongo class battleships were in the Nagumo forces. Japanese doctrine at the time was not that the battleships protected the carriers with their firepower. The main defense against an aircraft attack on the carriers was the maneuverability of the Japanese carriers. Having ships close aboard would restrict this. It was American doctrine that the larger ships, cruisers and, later, fast battleships would create a wall of AA for the attackers to fly through. This was much more effective, even at Midway.