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A naval force has command of the sea when it is so strong that its rivals cannot attack it directly. With command of the sea, a country (or alliance) can ensure that its own military and merchant ships can move around at will, while its rival's are forced either to stay in port or to try to evade it. Most famously, through the Royal Navy, Great Britain held command of the sea for long periods from the 18th to the early 20th century, allowing Britain and its allies to trade and to move troops and supplies easily in wartime while its enemies could not: for example, Britain was able to blockade France during the Napoleonic Wars, the United States during the War of 1812, and Germany during World War I.

During the age of sail, there were two primary countermeasures to another power holding control of the sea: smuggling, and privateering. Smuggling helped to ensure that a country could continue trading (and obtaining food and other vital supplies) even when under blockade, while privateering allowed the weaker power to disrupt the stronger power's trade. A more modern countermeasure, similar to privateering, was the use of submarine warfare by Germany during World War I and World War II to attack allied merchant shipping primarily in the Atlantic Ocean, Mediterranean Sea, and Baltic Sea.

During World War II, aircraft also became an effective countermeasure to command of the sea, since ships could not defend themselves well against air attack. The Battle of Britain was largely an attempt by Germany to eliminate the Royal Air Force, so that it would not be able to defend the Royal Navy from air attack.

Historically, many powers attempted to extend command of the sea into peacetime, imposing taxes or other restrictions on shipping using areas of open sea. For example, Venice claimed the Adriatic, and exacted a heavy toll from vessels navigating its northern waters. Genoa and France each claimed portions of the western Mediterranean. Denmark and Sweden claimed to share the Baltic between them. Spain claimed dominion over the Pacific and the Gulf of Mexico, and Portugal over the Indian Ocean and all the Atlantic south of Morocco (Hall, 148-9).

Gradually, however, countries agreed that the open seas should be free to all shipping in peacetime and to neutral shipping in wartime. Great Britain accepted the principle in 1805; Russia, in 1824; and the United States (tacitly) in 1894. Many treaties, including the Treaty of Versailles, have dealt with the open seas, and currently the United Nations Convention on the Law of the Sea formalizes this freedom.

• WE Hall, Treatise on International Law, 4th ed., 1895.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.