Seacoast defense was a major concern for the United States from its independence until World War II. Before airplanes, many of America's enemies could only reach it from the sea, making coastal forts an economical alternative to standing armies or a large navy. After the 1940s, it was recognized that fixed fortifications were obsolete and ineffective against aircraft and missiles. However, in prior eras foreign fleets were a realistic threat, and substantial fortifications were built at key locations, especially protecting major harbors.

Fort William and Mary was a colonial fortification in Britain's worldwide system of defenses, defended by soldiers of the Province of New Hampshire who reported directly to the royal governor. The fort, originally known as "The Castle," was situated on the island of New Castle, New Hampshire, at the mouth of the Piscataqua River estuary. It was renamed Fort William and Mary circa 1692, after the accession of the monarchs William III and Mary II to the British throne. It was captured by Patriot forces, recaptured, and later abandoned by the British in the Revolutionary War. The fort was renamed Fort Constitution in 1808 following rebuilding. The fort was further rebuilt and expanded through 1899 and served actively through World War II.