Liggett's services in the Philippines included setting up a staff ride in 1914 to study possible invasion sites on Luzon. He was assisted in this by his aide de camp, Captain George Marshall. The staff ride established that the most likely invasion route would be through the Lingayen Gulf and that this would be all but unstoppable unless the US dramatically increased its army and navy forces in the Philippines. In 1941, the Japanese invaded through the Lingayen Gulf, as the United States did in turn in 1945.