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 three 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."
"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)
This character's surname was formatted "Green" in the script of "The Savage Curtain", and in a barely legible graphic that appears in ENT: "In a Mirror, Darkly, Part II". However, the name was spelled "Greene" in the final draft script of "Demons".
Green's first name was given as "Phillip" in the aforementioned obscure graphic from "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.
The uniform worn by Steve Rankin in "Demons" was previously worn by a Mokra Order soldier in the Star Trek: Voyager episode "Resistance", by a Kradin in the episode "Nemesis", and by Steve Blalock in the Enterprise episode "Canamar". It was sold off on the It's A Wrap! sale and auction on eBay. 
Gene Roddenberry conceived of Green as "Mr. Green, who was something of a U.S. 'Hitler-racist' in the late 20th Century." Roddenberry described the character in this way in an initial story outline for "The Savage Curtain", which he wrote on 8 May 1968. (These Are the Voyages: TOS Season Three) Imagining how Green might have been conceived, Manny Coto commented, "There's so much history that has been hit upon in the original series all the way through Next Gen, and touched on in characters that are mentioned, like Colonel Green, and things like that, that are just kind of dropped in. And you know they're just dropped in by writers on a weekend, [...] you know, who are just like, 'Ah...' Gene Coon is just like, 'Oh, Colonel... Okay, we'll call him Colonel Green.'" ("Before Her Time: Decommissioning Enterprise, Part One: New Voices", ENT Season 4 Blu-ray special features)
A Star Trek fan himself, Phillip Pine was delighted to receive the role of Green, play the part, 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. Adolf 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.
Green proved extremely popular. Although Phillip Pine played many villains in television productions, he found Green to be the most often recognized. "I think Colonel Green is probably well-remembered because he was such an unmitigated bastard," Pine mused. "He had no redeeming qualities at all; none. The guy's smile was even sinister! He probably had bad teeth and they hurt. Everything about him was bad [....] My grandson told me that I should get a red jumpsuit [like he wears in 'The Savage Curtain']!" (Starburst Special #29, pp. 59 & 62)
One fan of Green was Manny Coto. "For some reason, I have a fetish for Colonel Green," he laughed. "I can't describe it. I've always found him a fascinating character, because it's this dark... It was like a Hitler. You know, in the original series, when they mention Colonel Green, it was like, ooh, the Federation had a Hitler in it. That's fascinating." ("Before Her Time: Decommissioning Enterprise, Part Two: Memorable Voyages", ENT Season 4 Blu-ray special features)
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." wbm 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." wbm Coto invented an extensive backstory for the character's appearance on Star Trek: Enterprise. "It was fun fleshing him out and creating a whole history, a background for him in these episodes," said Coto.  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. wbm Coto also considered Green being portrayed by Peter Weller, though he went on to instead appear as John Frederick Paxton in fourth season two-parter "Demons" and "Terra Prime". ("Terra Prime" audio commentary)
Green was featured in an ultimately undeveloped ENT story devised by Garfield and Judith Reeves-Stevens, also intended for the series' fourth season. In that narrative, Green was linked to an extremist political movement called the Optimum (which the Reeves-Stevenses took from their novel Federation). A heretofore secret connection between Green and Malcolm Reed's great-grandfather would have been established in the story. ("Observer Effect" audio commentary, ENT Season 4 Blu-ray special features) Also, Green's appearance in the undeveloped episode would have linked him to an emblem previously shown on a World War III uniform worn by Q in TNG: "Encounter at Farpoint", here used as Green's personal command emblem. A flag featuring the emblem was produced for "In a Mirror, Darkly", with the hope that it could be reused, but this never came to pass. 
The final draft script of "Demons" commented, "Greene is a classic firebrand, a boldly powerful speaker." By then saying, "The scene has a Triumph of the Will quality to it," the script indicated that Green was to seem like a Nazi leader.
As Green was to be represented only on a monitor in "Demons", the casting of the part for that reappearance took place after production on the episode wrapped. wbm The role had to be recast because Phillip Pine had died since appearing in "The Savage Curtain". To adopt the part, Steve Rankin underwent several physical changes. "They made me up, got my hair dark, gave me the pointy sideburns," he said. Like Pine before him, Rankin was also thrilled to portray Green, having been a Star Trek fan too. "That was a very, very cool thing to do," he enthused, adding, "but Col. Green was one of the worst men in the universe."