The maritime history of the United States is a broad theme within the history of the United States. As an academic subject, it crosses the boundaries of standard disciplines, focusing on understanding the United States' relationship with the oceans, seas, and major waterways of the globe. The focus is on merchant shipping, and the financing and manning of the ships. A merchant marine owned at home is not essential to an extensive foreign commerce. In fact, it may be cheaper to hire other nations to handle the carrying trade than to participate in it directly. On the other hand, there are certain advantages, particularly during time of war, which may warrant an aggressive government encouragement to the maintenance of a merchant marine.