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.