Japanese submarines were initially successful, destroying two US fleet aircraft carriers, a cruiser, and several other ships. However, following a doctrine that concentrated on attacking warships, rather than more-vulnerable merchantmen, the smaller Japanese fleet proved ineffectual in the long term, while suffering heavy losses to Allied anti-submarine measures. Italian submarines and one German submarine[2] operated in the Pacific Ocean, but never enough to be an important factor, inhibited by distance and difficult relations with their Japanese ally.