Colonel Phillip Green was a despotic militia leader on Earth in the 21st century during World War III. Green, known for his motto: "Overwhelm and devastate," was notorious for striking at his enemies during treaty negotiations. (TOS: "The Savage Curtain")
Barely two years after a cease-fire had ended the war, he ordered the euthanasia of hundreds of thousands of radiation-sickened Humans so they would not pass on mutations to future generations. Green outlined his rationale by speaking to a crowd about how they must "reject the impure and cast it out." This act was met with mixed feelings by survivors, many regarding him as nothing more than a genocidal madman. Others saw him as a pragmatic visionary who "humanely" euthanized radiation-afflicted individuals, preventing those afflicted and their descendants from passing on deadly mutations and the suffering that would come with that. (ENT: "Demons") This controversial and contentious posthumous view of Green resulted in heated debates about him and the impact of his policies for many years; among the students of Green's teachings, years afterward, John Frederick Paxton made use of his ideologies to lead a group known as Terra Prime in an unsuccessful bid to expel all non-Humans from the Sol system in 2155. (ENT: "Terra Prime")
"Overwhelm and devastate."
"We must reject the impure and cast it out."
"This is not a time for timidity and second guessing. We cannot afford to doubt ourselves."
"To be human is to be pure."
"I have no quarrel with you, any more than you have with me!" (Excalbian duplicate)
"Men don't talk peace unless they're ready to back it up with war." (Excalbian duplicate)
"History tends to exaggerate." (Excalbian duplicate)
Green was played by actor Phillip Pine in TOS: "The Savage Curtain". A Star Trek fan himself, Pine was delighted to receive the role and be associated with it. "The character of Green was well written," he remarked. "There were little or no changes required by the director, Herschel Daugherty, or myself to highlight Green's Machiavellian mind. Adolph Hitler was discussed at one point and I was certainly influenced by that man's character in playing the role." (Starlog #130, p. 76) The tone in which he addressed Surak as, "...Vulcan!" in the episode may have been some insight into Green's xenophobic attitude towards aliens.
Originally, Manny Coto intended to use Green as a villain in the Augments trilogy in season four of Star Trek: Enterprise. While working on the story, he said, "I want to do a very harrowing 'Colonel Green' arc. I'm determined to get 'Colonel Green' into this show."  Coto also thought Green fit well into the plot with the Augments, later admitting, "I always thought that he was an interesting character to bring to life and flesh out. But the implication was that he had something to do with the Eugenics Wars as well." After he learned that Brent Spiner was interested in a role on the series, Coto changed the story arc to accommodate Spiner. "For a little while," Coto explained, "I even toyed with the idea of him playing Colonel Green." Coto instead created the character of Arik Soong for Spiner to play. 
Green's appearance in Enterprise's Augments trilogy would have featured the logo of the Fourth World military (as shown in TNG: "Encounter at Farpoint"), here used as Green's personal command emblem. (citation needed • edit) After Green was removed from the Augments arc, the only other appearance of that emblem was in "In a Mirror, Darkly".
Green's first name was given as "Phillip" in a barely legible graphic seen in "In a Mirror, Darkly, Part II". The name was likely chosen in reference to the first name of the character's original actor, Phillip Pine.