When the Star Wars Prequel Trilogy began, it was not immediately clear how it would have been possible for the Jedi to go from a far-reaching order of interstellar peacekeepers to a forgotten people thought of as little more than a myth, all in the span of three decades.
We partially got an answer with Revenge of the Sith and Palpatine’s execution of Order 66 though, a protocol hidden within the Clones’ programming that named the Jedi as traitors to be subjected to summary execution, resulting in what became known as the Great Jedi Purge and the rise of the Empire. It was never clear why the order was given such a designation, but several theories have been put forward.
Executive Order 9066, which was signed into law by President Roosevelt in March 1942, gave the Secretary of War the power to designate any area of the US a military zone, paving the way for concentration camps in which over 100,000 people of Japanese ancestry, mostly American citizens, were detained. Although with a different objective than Order 66, it has a thematic similarity in arbitrarily declaring all people of a certain demographic to be traitors to a nebulously defined cause, and affords those in power the rights over their lives.
Other number plays suggest Order 66 is intended to invoke 666, well-known as the Number of the Beast from the Book of Revelation, while another possibility is the numbers representing FF to stand for Friendly Fire, as the order resulted in the Clone Troopers turning on their allies and commanders. Similarly, it could be a variation of 88, often used as neo-Nazi code for HH, abbreviating Heil Hitler, which is an appropriate reference given the Third Reich imagery with which the Empire is awash.
Of course, it’s also possible that Order 66 doesn’t actually mean anything, as it was, after all, a secret addition, so calling it something meaningful like Red Blade or Dark Two would have set some alarm bells ringing even for the arrogantly complacent Jedi, who could have put an end to Palpatine’s plan before it began. But as far as theories go, each one of these has its merits.