Command of the sea

A naval force has command of the sea when it is so strong that its rivals cannot attack it directly. Also called sea control, this dominance may apply to its surrounding waters (i.e., the littoral) or may extend far into the oceans, meaning the country has a blue-water navy. It is the naval equivalent of air supremacy.

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 rivals are forced either to stay in port or to try to evade it. It also enables free use of amphibious operations that can expand ground based strategic options. Most famously, the British Royal Navy 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 (the importance of which is reflected in the famous British patriotic song, "Rule, Britannia!," which contains the exhortation, "Rule Britannia! Britannia rule the waves", even if this was not the poem's original subject). 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. In the post-World War II period, the United States has had command of the sea.

Few navies can operate as blue-water navies, but "many States are converting green-water navies to blue-water navies and this will increase military use of foreign Exclusive Economic Zones [littoral zone to 200 nautical miles (370 km)] with possible repercussions for the EEZ regime."

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.

...
Wikipedia