I believe 伏敵 is a shortened form of 敵國降伏. At Hakozaki Shrine in Fukuoka, a tower gate with the inscription 敵國降伏 is informally referred to as 伏敵門 (門 meaning gate).
For 敵國降伏, the shrine itself uses the translation “may the enemy nations prostrate themselves [in defeat]”.
敵國降伏 comes from Imperial calligraphy, apparently first written down by Emperor Daigo in 921, which are included in the shrines treasures. It is associated with Mongol invasion as the shrine was burned during attacks in 1274. It was rebuilt and with the assistance of Emperor Kameyama a plaque with the inscri