Passage 2

Aircraft Carriers are a powerful and extremely flexible part of a nation’s defense. A carrier is flexible in a way that armies and air forces are not because it can influence s merely by being seen. The power of an aircraft carrier can act as a visible deterrent (威慑物) to enemy aggression. A carrier is also flexible because it operates on the ocean. Unlike a land air base, no other country has to be consulted about where a carrier can go and what it can be used for because most of the ocean is considered international water and not part of any one country. This allows carriers to move from one trouble spot to another quite easily. Carriers operate with a number of support and supply ships; this flotilla (驱逐舰队) of ships is referred to as a carrier battle group. Since World War II (1939-1945), carriers have primarily been used to influence world s and to support troops ashore. When the Korean War (1950-1953) broke out in June 1950, United States Navy careers were the only military airpower immediately available to engage the enemy. U.S. Air Force units were unable to respond for several months. Aircraft carriers were also the first American units to arrive on the scene during the Vietnam War 1959-1975)and the Persian Gulf War (1990-1991). While carriers are important, they are also quite expensive, so there are only a few patrolling 巡逻) he world’s oceans today. Approximately 40 carriers of all types are currently in use or are being built. Of these, over half are in service in the U.S. Navy. A typical American nuclear powered aircraft carrier can cost well over $ 4 billion and take five years to build.

From the second paragraph, we know the author tried to tell us ______ .