The Enterprise has barely begun its mission to investigate the mysteries of Farpoint Station when it confronts a barrier that looks like a steel mesh fence in space-- a “solid object” “or an incredibly powerful force field.”
The Enterprise stops, and in a flash of light a strange being appears, dressed as an Elizabethan ship’s captain, with armor and ruffles and a cocked hat with a feather: it is Q (“We call ourselves the Q, or thou mayst call me that…”)
Q tells Picard that “thy kind has infiltrated the galaxy too far already” and they are “directed to return to your own solar system. Go back whence thou came’st.”
When an ensign attempts to draw a phaser on Q, he blasts him with frigid air and essentially deep-freezes him. Picard shows Q the phaser (which today looks a lot like an Ipod) and says it was set on stun. “He would not have injured you.” Q replies slyly, “ Knowing humans as thou dus’t, Captain, would thou be captured helpless by them?”
In a minute or so, Q is going to figure out he’s misjudged the historical period this Enterprise is in, but the choice of the era defined by Q’s clothes may not have been arbitrary. At roughly the time of sea captains dressed in what Q was wearing, seafarers were exploring the globe, and conducting scientific and navigational research—but they were doing more than that. Sailing ships from various nations were competing for trade routes, and fighting each other. They were the representatives of not only nations but business compacts, and so they were seeking products and land in new places. They were often the first wave of military conquest.
For example, Christopher Columbus discovered the Americas, but he began the colonization of Native Americans immediately, and and took some as slaves back to Europe. The conquests of the Americas also began with missionaries attempting to convert Natives to Christian and European ways, though always as subservient. The conquest and colonization by European nations over the next several hundred years usually began with heavily armed ships, eventually carrying heavily armed soldiers. They sought the wealth of these lands, forcing native populations to mine precious metals and grow foodstuffs, and later to supply raw materials for the industrial revolution. They soon began shipping their captives elsewhere as slaves.
Entire continents were transformed by the European “explorers,” with transformations of the natural and cultural landscape, and human death tolls in the millions, as entire peoples were enslaved and exterminated. And this all began with the mariners, the ship captains and their scientists, those who Buckminster Fuller called the Great Pirates.
For centuries, these were the models of “explorers”—they were also invaders, conquerors, enslavers. These were the primary reason for the Prime Directive, which is less about "non-interference" than non-conquest, anti-empire.