Fort Frank was heavily engaged in the Japanese invasion of the Philippines. On 31 January 1942 the fort's mortar battery bombarded mainland positions in the Pico de Loro Hills that the Japanese were emplacing artillery in. The 75 mm guns were also able to engage mainland targets. The Japanese began bombarding Fort Drum and Fort Frank on 6 February 1942. Fort Frank was vulnerable in another way: its normal water supply was from a dam's reservoir on the Japanese-held mainland. On 16 February the Japanese discovered this and removed part of the pipeline near the dam. Although the fort also had a distillation plant to provide fresh water, this consumed fuel that was also needed for the gun batteries' generators that powered the ammunition hoists. The fort's commander ordered the distillation plant started, but also directed a 15-man team to attempt to restore the pipeline on the 19th. They successfully engaged a Japanese patrol but could not get to the pipeline. Eventually another party repaired the pipeline on 9 March. On 20 March 1942, thirty-four soldiers were killed by Japanese artillery when a round ricocheted into a tunnel at Battery Crofton.