"The U.S. Navy - with the world's largest and most powerful operational fleet - has acquired a small Swedish submarine to help it to learn how to fight similar craft in coastal or littoral waters. Playing the aggressor role, the Swedish sub Gotland has demonstrated that antisubmarine warfare (ASW) against non-nuclear submarines is extremely difficult. The U.S. Navy has no non-nuclear subs except the research boat USS Dolphin (AGSS-55)."