The United States of America is a land power – actually, to be more accurate, a continental power – that has had its development and commerce shaped fundamentally by water even if most Americans were unaware of this fact. There was, however, a small group of political and military leaders who understood both the importance of the sea and its flexibility as a medium of power. These two factors – the importance of water and a general adaptability of the U.S. military services that fought on and near it – are major considerations in explaining U.S. strategies and experiences in the Second World War.

Throughout American history, natural bodies of water had been highways for commerce and migration, and, to a much lesser extent, defensive barriers. That changed on 7 December 1941. While maritime factors had led the United States to war against France in the undeclared Quasi War of the 1790s, against Great Britain in the War of 1812, and against Germany in the First World War, the Second World War was different. The Imperial Japanese Navy had shown that weapons technology now made it possible for foreign powers to threaten the security of the American homeland.(The last foreign power to do so – the United Kingdom in 1812 – had bases in Canada that negated the barrier of the Atlantic even before the war started.) It was also within the resources of Nazi Germany and Imperial Japan to damage in a systematic way the commerce of the United States.

When the United States entered the war, it faced a strategic situation that several peacetime decisions had shaped.