The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.

A marine is a member of an infantry force that specializes in naval operations such as amphibious assault. In some countries, a marine force is often part of a navy, but can also be under army or independent command.

Historically, tasks undertaken by marines have included providing protection from war while at sea, reflecting the pressed nature of the ships' company and the risk of mutiny. Other tasks would include boarding of vessels during combat or capture of prize ships and providing manpower for raiding ashore in support of the naval objectives. Marine elements would also contribute to the campaign ashore, in support of the military objective.

With the industrialization of warfare in the 20th century the scale of landing operations increased; thus brought with it an increased likelihood of opposition and a need for co-ordination of various military elements. Marine forces evolved to specialize in the skills and capabilities required for amphibious warfare.

In the earliest days of naval warfare, there was little distinction between sailors and soldiers on a warship, as the oarsmen of Ancient Greek and Roman ships had to be capable of fighting the rowers of opposing ships hand-to-hand; though hoplites began appearing on Greek ships specifically for the boarding of enemy ships. The Roman Navy's two legions, I Adiutrix and II Adiutrix, were among the first distinct naval infantry units.