The Arkadian Civil War is an important event in The 100. The conflict consisted of two factions, one lead by Chancellor Charles Pike and another lead by former Chancellor Marcus Kane. As of the episode "Fallen", everyone in Pike's faction was either killed, turned over to Kane's faction, or became chipped. Most of the people who survived the conflict later joined a resistance to defeat A.L.I.E.. The conflict was resolved after members of Kane's faction succeeded in Pike's capture, and the latter was taken by the Grounders to Polis.
After the battle for the Mountain, the alliance with the Coalition fell apart and Arkadia maintained a fragile truce with Polis, they had to be cautious with all their decisions fearing repercussions, thus unable to make full use of Mount Weather supplies and facilities. Meanwhile, Farm Station survivors continued to be hunted in Azgeda lands despite the truce.
After three months of relative peace in Arkadia, Chancellor Griffin tried to pass the chancellorship pin back to Kane, however, they decided it is time to hold an election instead. Marcus Kane and several others finally came into contact with Farm Station survivors, four months after landing on Earth. Upon returning to camp, Pike moved half his people into the now-abandoned Mount Weather.
Upon settling into the Mountain, however, an assassin from Azgeda blew up Mount Weather, killing 49 people and leaving it uninhabitable (due to the loss of the Mountain resources Arkadia will be starving in less than a year). As a result of this most recent attack, many Sky People have lost faith in an alliance with the Grounders, with whom they have been in state of the perpetual conflict from the moment they landed on Earth.
The next day, the election was held. Pike won the election, allowing him to perpetuate an attack on the Coalition force outside Arkadia, massacring 300 Trikru warriors who sent to protect Arkadia from the Ice Nation.
In Watch the Thrones, Pike tries to convince the leaders of Arkadia that they need to retaliate against the grounders, however, Kane and Abby disagree. This causes Pike to band together his group of people from Farm Station to rebel against the leaders, convincing Bellamy Blake to help him in doing so. Pike and his group steal weapons and attempt to leave Arkadia to kill 300 Grounders surrounding the camp, but are caught and arrested. The next morning, Pike is appointed the new Chancellor of Arkadia. His first actions are to pardon himself and the others, reject the brand that made them the 13th clan, and continue onward with their original mission (killing the Grounders surrounding Arkadia)
In Hakeldama, Pike and his regime sought out and kill the 300 Grounders, but leaving one alive to deliver a message to the Grounders, which was that the Sky People no longer wish to be at peace with the Grounders. Upon their return to Arkadia, Pike gives a speech to all Sky People saying that they own the land now, and will not allow any Grounders to take it from them. Kane and Abby try to take control of the situation, however, most of the Sky People agree with Pike that the Coalition should be rejected. Later that day, Bellamy is greeted by Clarke in secret, trying to convince him that he shouldn't be on Pike's side, and should go back to who he was. Bellamy ignores her and attempts to arrest Clarke, but is thwarted by his sister, to which the two then escape Arkadia.
In Bitter Harvest, Hannah Green, Shawn Gillmer and a few others are sent on a scouting mission. During this time, Octavia Blake is seen spying on them under Kane's orders. Eventually, a young Grounder boy appears and shows no hostility, but Hannah gives the order anyways and tries to have the boy killed. Octavia manages to grab and save the boy, while Hannah, Shawn and the rest return to Arkadia. Upon their return, Kane finds out that Pike is planning to attack a nearby Trikru Village. Kane informs Octavia to warn the villagers, and she makes her way to the village. Upon arriving, the Grounders show hostility and attack Octavia. However, the Grounder boy she had saved from before appears, saving her. However, the grounder chief keeps Octavia captive until the Sky People arrive for the invasion.
Later that night, Bellamy, Monty, Monroe, Hannah and a few others arrive at the Grounder village to stage an attack, but are thwarted upon entering the camp, when the villagers shoot arrows at pyres, causing toxic gas to come flow out, killing Monroe and another Arkadia guard. Octavia is then kidnapped by the villagers and taken to Polis. Later, Hannah, Pike, and Bellamy are conferencing and determine that Kane is responsible for Octavia having warned the grounders of the attack and resulting in the death of Monroe though they have no evidence to convict him as guilty.
In Terms and Conditions, Kane has formed a full on resistance including Harper and Miller who listen in on Pike's council meetings. Kane wants to resolve the situation peacefully but Harper disagrees saying they should "Shocklash [Pike's] fascist ass and hand him to the grounders". Kane tries to negotiate with Pike who is aware that Kane is up to something but is unable to get proof. Kane and Sinclair develop a plan to make sure the rover doesn't work but Sinclair is caught and put in lockup, it is at this point Kane realizes the situation can no longer be resolved peacefully. While Bellamy is trying to get information on Kane from Sinclair, Lincoln starts a fight setting the plan in motion. Pike who is aware this was all part of Kane's plan is hit with Kane's stun stick and put in the rover. Kane drives off with the rover in the attempt to bring Pike to the Grounders but is stopped by Bellamy. Later that evening, Kane is sentenced to death for his crimes.
In Stealing Fire, Kane, Sinclair, Lincoln, and the other Grounder prisoners are in lock-up and Pike sentences Sinclair and Lincoln to death alongside Kane. Harper and Miller are planning to help them escape before they are approached by Monty and Bellamy who want to help but Harper and Miller refuse. Miller discovers a bug in his guard jacket planted by Bryan. Miller approaches Bryan and confronts him about the bug. Later, Abby visits Kane before his execution, he begs her not to help him escape since he knows she would be condemned to death too. Back in lock-up Pike comes to retrieve the prisoners, Monty and Hannah are monitoring the radio and hear that "S" intends on helping the prisoners escape (S being Octavia.) Bryan informs Pike that they should wait it out since Octavia has likely told them where the prisoners are being taken. The prisoners are taken into a room and Pike orders two guards on watch who will shoot to kill if anyone tries to escape. The guards, however, are drugged and the prisoners hide under the floorboards of the room. Pike calls for medical to attend to guards which brings in Miller and Abby. Harper is monitoring the exit and radios Octavia that the exits are not clear. Monty who is listening in with his mother creates a distraction calling all the guards to the main gate. The prisoners are then given to opportunity to escape however, Abby chooses to stay behind. Pike then threatens to kill the other Grounder prisoners if the prisoners do not turn themselves in. Lincoln then surrenders himself allowing the others to escape sedating Octavia after giving her a goodbye kiss. Later as Sinclair, Kane, Bryan, Miller and Octavia escape they watch Lincoln's execution which devastates Octavia but upon looking at Pike her devastation soon turns to rage.
In Fallen, Kane, Sinclair, Harper, Miller, Bryan, and Octavia have returned to Lincoln's cave. Octavia finds her brother and beats him mercilessly blaming him for Lincoln's death. Back in Arkadia, Monty's mother informs him that Pike knows of his betrayal and advises he leave immediately. It is later discovered that she turned him in. Monty radios Bellamy and Kane responds Monty informing them that Pike knows he helped them. Kane asks him if he can meet at the dropship and Monty says he can. Kane insists on going to the dropship alone but Octavia gives him an ultimatum and they also bring Bellamy as a prisoner. Bellamy, Octavia, and Kane arrive at the dropship and Monty exits Pike following holding a gun to his head. A few of Pike's guards attack and Kane drops his weapon surrendering. Pike tells Bellamy he needs to reconvince him he is on his side and Bellamy complies saying the other are in a cave that is north of the dropship. Bellamy is then leading Pike who now has Kane and Octavia prisoner and makes a deal with him that Octavia will be spared. Pike realizes they are approaching the blockade line and Bellamy lies that Kane selected this location since he believed Pike wouldn't go this far. However, Bellamy turns on Pike and turns a gun on him. Bellamy calls for the grounders in the blockade informing them that they have Pike who they come to retrieve. In the scuffle, Octavia kills Shawn Gilmer in vengeance and the grounders take Pike. Before the grounders leave with Pike, Kane asks to accompany them to the Polis.
After Pike was defeated, Kane escorted him to Polis with other grounders to take to the new commander. When they arrived, however, A.L.I.E. had brainwashed most of the Grounders/Sky People to fight for her. The remaining members of Kane's faction with Charles Pike, formed a resistance that fought against A.L.I.E to destroy her, and succeeded in doing so, however, Pike was killed by Octavia as revenge for Lincoln's death.
Lincoln – Executed by Pike for treason after causing a prison riot with the other prisoners as a distraction for Kane to capture Pike.
Additionally, 49 Sky People in Mount Weather and 299 Trikru warriors sent by Lexa were killed prior to the conflict and contributed to the divide among the Sky People. 2 Grounder messengers were also shot by Bellamy in Terms and Conditions.
Pike (to Arkardia residents): "This morning on a muddy field, our people paid tribute to those who have been taken from us by sending a message to the Grounders. This land is ours now. Resist and you will be met by force. Fight and you will be greeted by death. Today is a new beginning. Mark it down. Remember it. Just like the Grounders will remember it."
Kane: "Wake up. You attacked an army that was here to help us. You murdered innocent people. Is that who you are now? You thought you couldn't live with yourself after Mt. Weather? Well, you just started a war that'll kill us all."
Pike: "We can't do what's needed to defend this camp if every order I give is leaked before it can be executed. It's an old saying, but it's true... the walls have ears... and we can't afford any more assumptions about who's a friend and who isn't, not your oldest acquaintance, not your husband, wife, or lover. We're fighting two wars now, and the more dangerous one is here inside this camp. We can't prove it yet, but Kane and his accomplices passed information to Octavia. I know none of you signed up to investigate your neighbors, but Monroe and Lacroix died because the traitors in this camp sold them out to the Grounders. Whoever did that will be hunted down and exposed for what they did to their own, for what they did to us. Now you get whatever resources, whatever personnel you need to make that happen."
Pike: "As you know, the terms of martial law and of the Exodus Charter give me both latitude and ultimate authority in determining your punishment. Do you have anything to say for yourself?"
Kane: "I admire your adherence to the rule of law... I really do... but these are times when we have to look beyond the rules... to realize they were established to serve a world of the past, not of the future. I beg you one last time... to see the world as it is, not as it was or as you want it to be."
Pike: "And I hope you believe... that if I thought for one minute that turning myself over to the enemy would secure the safety of our people... I would do it.. but only a deluded man would believe that. And only a guilty man would try and tell him so. Marcus Kane, for the crimes of treason, kidnapping, and attempted murder, I hereby sentence you to death."
Pike: "Kane's actions put this whole camp, everyone's lives at risk today. He costs us lives in his misguided zeal to appease our enemy, people you knew. Your friends died because of him. Am I making him an example? You're damn right, and I hope his execution makes it clear once and for all where people's allegiances need to lie."