As naval power grew in Europe during the 16th century, some of the not-so-powerful nations had a problem. The Dutch, the French, and the English were tearing each other apart on the high seas, and sometimes the action got too close for comfort. What was a country like Denmark or Italy to do when hostilities were breaking out just off their coasts? How could they maintain their neutrality and not get dragged into the mess? Around the Mediterranean and the North Sea, these countries set about establishing the first buffer zones—what we now call territorial waters. Within these buffers, aggressor nations (or more often, their hired privateers) had to chill out.
In southern Europe the established buffer was the rather subjective “distance of a cannon shot from shore,” an easily understandable measure but a little vague and subject to change as technology improved. In northern Europe, Denmark (which at that point owned Norway, Iceland, and Greenland, and was also quite concerned about foreign powers fishing near those lands) championed a uniform distance from the shore that was not based on artillery. That sounds logical now, in the era of long-range missiles, but without any established reason for the distance, it was susceptible to political winds. Over the years Denmark’s official limit fluctuated from a mile to twenty miles and back again, ebbing and flowing with the Danes’ political sway.
By the end of the 18th century, when America got involved, these varying views on territorial waters had coalesced into a generally accepted distance of three nautical miles, otherwise known as one league. This was about the distance of a cannon-shot at the time, but stated in actual distance, which was helpful. In 1793, as the French Revolutionary Wars pulled France and England into conflict, the new US of A was worried about maintaining its neutrality and established its own buffer. Like Denmark before it, different political leaders in the early US would try to impose wider limits (apparently Jefferson thought it should extend to the Gulf Stream, since it was a “natural boundary,” though at points the Stream is many miles out to sea, and its location changes constantly), but the young country was ultimately convinced to use the European norm, and the three mile limit became law.
At this point you might be saying to yourself that, while all this international maritime legal history sure is scintillating, it doesn’t seem to have much to do with liquor. And it didn’t, until Prohibition. The 18th Amendment created a huge black market for alcohol, much of which had to be smuggled into the country. Rum runners would drop anchor in international waters, then make runs from ship to shore under cover of darkness. Coast Guard attempts to put a stop to the practice led to court cases, law changes, and diplomatic incidents.
In 1922, the owners of the British-flagged ship Grace and Ruby went to court after their ship was confiscated four miles off the coast of Massachusetts. They said that while yes, they were smuggling liquor, they were in international waters when they were stopped, so the seizure was illegal. The court ruled that the presence of some of the crew and one of the ship’s dinghies in US waters (ferrying that illicit booze to shore) constituted the “constructive presence” of the ship itself. Score one for Legal Logic.
The Brits weren’t too happy about this, but Congress doubled down, codifying that legal logic in a new law that gave the Coast Guard the right to search any ship within three leagues – three times the standard limit. Over the next several years, more and more ships flying international flags were stopped and seized off the American coast. Even ships that just had alcohol on board for passengers and crew were targeted – in 1923 New York authorities confiscated all the liquor from a British luxury cruise ship that was anchored there. The hubbub that caused brought about a whole new treaty between Britain and the US. The treaty allowed ships to have alcohol on board while they docked in American harbors, as long as they kept it under lock and key. In return, Britain agreed to allow the Americans to chase down suspected rum runners beyond the 3-mile limit.
The US signed these kinds of treaties with sixteen different countries during Prohibition, and by 1933 the government enjoyed far greater authority over coastal waters than it did before the Noble Experiment began. Other countries wanted the same, and soon the universal three mile limit was an endangered idea. Through the twists and turns of international politics, the US actually ended up the primary defender of the limit later in the century, mostly because we didn’t want to have to ask permission to send our warships and submarines through some of the world’s narrower straights. In the end, other concerns (namely the original ones from the 16th century, national security and fishing) proved more pressing, and in 1989 the US became the 105th nation to sign on to a new standard of twelve nautical miles. That’s where we are today, with, you’ll be unsurprised to hear, a whole bunch of exceptions and fine print.
 International law in this era was a fuzzy affair, and there was nothing that said a country couldn’t just expand its territorial waters like this – it was just sort of frowned upon, seen as protectionist and unfriendly.
 Instead of just allowing for a wider buffer, the treaty based it on the speed of a ship – any ship that was within one hour of shore could be searched, so a ship capable of ten knots, for example, could be searched up to ten miles off shore.
Swarztrauber, Sayre Archie. “The Three Mile Limit of Territorial Seas: A Brief History.” American University dissertation, 1970. Available on ProQuest.
Fulton, Thomas Wemyss. “The Sovereignty of the Seas.” William Blackwood and Sons, Edinburgh: 1911. Available on HathiTrust. (Jefferson Gulf Stream story from p. 575)
Spinelli, Lawrence. “Dry Diplomacy: The United States, Great Britain, and Prohibition.” Rowman and Littlefield, Lanham, MD: 2008. Portions available on Google Books.
Kent, H.S.K. “The Historical Origins of the Three-Mile Limit.” The American Journal of International Law, Vol. 48, No. 4 (Oct. 1954), pp. 537-553.
Newsom, David D. “Why the Three-Mile Limit Sank.” The Christian Science Monitor, January 26, 1989.