Captain James T. Kirk is known as Star Trek's resident rule breaker, but how many times did he actually violate Starfleet's Prime Directive? Acting as the guiding principle for space exploration for centuries, the Prime Directive was Starfleet's most important law and was designed to protect underdeveloped cultures from outside influence. Though the rules were much faster and looser during the Star Trek: The Original Series era, the Prime Directive was still broken on numerous occasions and for various reasons. Though justifications were always made, Captain Kirk showed quite frequently that he had little regard for Starfleet's golden rule and would instead do whatever he thought was right.
While it had dozens of subsections and additional strictures, the most important part of Starfleet's Prime Directive was its non-interference rule that kept Federation personnel from even making themselves known to pre-warp planets. Even when not revealing the intergalactic nature of their missions, Kirk's outings often involved breaking the Prime Directive by interfering with the natural development of a world or someway manipulating their government. Though episodes like "Patterns of Force" and "The Paradise Syndrome" involved breaking the Prime Directive, extenuating circumstances exonerated Kirk. Despite those exceptions, Kirk still offered his own loose interpretation of the rules when dealing with alien civilizations. Here's every time Kirk violated the Prime Directive.
Miri (S1, E8)
Though Star Trek: Enterprise later revealed the tragic origins of the Prime Directive in canon, the rule itself wasn't mentioned in Star Trek: The Original Series until season 1, episode 21, "Return of the Archons." Nevertheless, the rule existed during the events of episode 8, "Miri," and Captain Kirk's actions constituted a subtle violation of the rules. In the episode, the Enterprise answered a random distress call that led them to a planet where children are the only survivors of a plague. Kirk rescued the children and had them vaccinated for the deadly disease, but in doing so he actually interfered with the natural development of an alien world.
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While the particular subsection of the Prime Directive wasn't fully explored until Star Trek: The Next Generation, it was technically a violation of the law to help the children in "Miri." Even beaming down to the planet without assessing their capabilities was illegal, and their assistance topped it off. The Prime Directive drew inspiration from real laws, and as such the policy of non-interference has its good side and bad. Starfleet was prohibited from hurting societies by getting involved, but it was just as illegal to save the children since it was their planet's natural development for them to succumb to the plague.
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Return Of The Archons (S1, E21)
Despite being the episode that introduced the idea of the Prime Directive into Star Trek, "Return of the Archons" was a violation of that same principle. Finally responding to the loss of a Federation vessel over a century earlier, the Enterprise crew discovered a planet ruled by a cruel computer called Landru. Though the loss of the USS Archon had caused a turn in the development of the planet, Kirk and his crew's arrival made things worse. By destroying Landru's computer, the captain, in the unmistakable Kirk style, completely changed the trajectory of their civilization, though he attempted to justify it by calling them "stagnant."
A Taste Of Armageddon (S1, E23)
"A Taste of Armageddon" presented Captain Kirk with a Prime Directive challenge, and he chose to simply break the rules instead of losing the game. While transporting a diplomat to make important headway on Eminar VII, the Enterprise fell victim to the planet's computerized proxy war. Obviously, Kirk did the right thing by not simply allowing his crew to be killed, but it was the destruction of the planet's war computer that violated the Prime Directive. The symbolic gesture wasn't Kirk's decision to make, and though it possibly resulted in peace between the worlds, it was a heavy-handed decision that Starfleet had attempted to curb with the Prime Directive.
The Apple (S2, E5)
Hardly one of the best Star Trek: The Original Series episodes, "The Apple" again featured Kirk's stagnation excuse as a reason to violate the Prime Directive, though no such provision was written in the rules. On the planet Gamma Trianguli VI, the Enterprise crew made with a race of people who lived to serve Val, a large machine. Making simple with the natives violated the Prime Directive. The destruction of Val made things worse. The people's needs were met by Val, and they seemed content to live their lives. The elimination of Val allowed them to self-determine, but Kirk made that decision for them without their consent.
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Friday's Child (S2, E11)
The best Klingon episodes often presented them as a meddlesome group of ruffians who interfered in alien cultures with little regard. In the season 2 episode "Friday's Child," the Enterprise traveled to Capella IV to negotiate a mining contract but the crew was forced into the planet's political game by the Klingons. How the Capellans, a presumably pre-warp civilization, came to align themselves with the Federation wasn't revealed, but that wasn't where the Prime Directive violation occurred. By preventing the natural succession of leaders when saving Eleen and her unborn child, Kirk interfered with the development of the society, which was illegal even after they made first with Starfleet.
A Private Little War (S2, E19)
While trying to make a comment on the Vietnam War, the Star Trek season 2 episode "A Private Little War" managed to present one of Kirk's most divisive decisions as captain. When the Enterprise visited the primitive people of Neural only to find that one faction had been armed with weapons by the Klingons, Kirk immediately chose to "even the odds" by arming the other side with similar weapons, a gross violation of the Prime Directive. It could be argued that the Prime Directive was out the window because of Klingon interference, but Kirk's brash decision led to an all-out escalation of deadly tension between the factions.
Star Trek IV: The Voyage Home (1986)
The temporal Prime Directive wasn't mentioned during any of Captain Kirk's five-year missions on the Enterprise, but Star Trek IV: The Voyage Home was stuffed with violations of the rules. Arriving back on Earth after Star Trek III: The Search for Spock, the Enterprise crew was dispatched to the 20th century to find whales. Acting on Starfleet's orders, Kirk was off the hook for the timeline manipulation in The Voyage Home by stealing the whales, but his interference with Dr. Gillian Taylor violated Star Trek's Prime Directive unnecessarily. Also, Scotty acted under Kirk's direct orders when he violated the Prime Directive by revealing the formula for transparent aluminum.