Following Colonel Evans Carlson's diversionary Makin Island raid of August 1942, the Japanese command was made aware of the vulnerability and strategic significance of the Gilbert Islands. The 6th Yokosuka Special Naval Landing Force reinforced the island in February 1943. In command was Rear Admiral Tomonari Saichirō (友成佐 – 郎), an experienced engineer who directed the construction of the sophisticated defensive structures on Betio. Upon their arrival, the 6th Yokosuka became a garrison force, and the unit's identification was changed to the 3rd Special Base Defense Force. Tomonari's primary goal in the Japanese defensive scheme was to stop the attackers in the water or pin them on the beaches. A tremendous number of pill boxes and firing pits were constructed, with excellent fields of fire over the water and sandy shore. In the interior of the island was the command post and a number of large shelters designed to protect defenders from air attack and bombardment. The island's defenses were not set up for a battle in depth across the interior. The interior structures were large and vented, but did not have firing ports. Defenders were limited to firing from the doorways.