自古以来,在发生战争或紧急态势的情况下,各国海军都会安排从事贸易活动的商船加入海军现役。在20世纪的海战中,多数国家将大型商船改装为武装商船,用于袭击敌方商船队和保护己方商船队,或是在拖网渔船上加装扫雷或反潜设备。进入21世纪,一些面临严峻资金短缺的国家海军仍然要担负大量繁重的日常任务,如保卫本国专属经济区(EEZ)、保护自然资源、打击毒品和其他违禁物品走私活动、拦截偷渡者以及应对海 Since ancient times, in the event of a war or emergency, navies of all countries will arrange trading ships to join active naval service. In the 20th century naval warfare, most countries converted large merchant vessels into armed merchant shipping to attack enemy merchant shipping fleets and protect their own merchant shipping fleet, or to install demining or anti-submarine equipment on trawlers. In the 21st century, some navy countries facing serious shortage of funds still have to undertake a large amount of heavy daily tasks such as defending the EEZ, protecting natural resources, cracking down smuggling of drugs and other prohibited substances, intercepting stowaways and coping with the sea