The Frontier War, also referred to as the Frontier Conflict, is the collective term for the conflicts waged between the Interstellar Manufacturing Corporation, Frontier Militia and various mercenary, pirate and other private military groups on The Frontier. This includes all the events depicted in Titanfall, Titanfall 2, Titanfall Online, Titanfall: Assault and the now-cancelled Titanfall: Frontline.
The initial Frontier settlers were IMC veterans, given money and land on the Frontier due to the IMC's so-called "G.I. Bill". Because of a series of wars back in the Core Systems, the IMC pulled their support of the initial colonists due to decreasing favour back home and to refocus their efforts. By the time the IMC returned several decades later, claiming eminent domain over the Frontier and its resources, a thriving civilization had emerged separate from Earth and its oversight, and much of the population had no intention of submitting to corporate control, not when they had done so well for themselves on their own. Ultimately, a clash of ideals was inevitable, and the Frontier's population found themselves in conflict with the IMC.
The Titan Wars
The Titan Wars were a series of conflicts waged between the IMC and Frontier settlers around fifteen years before the Battle of Demeter. The details of these wars are not known, except that the victory was likely an IMC one due to their continued control of the Frontier.
At the end of the Titan Wars, James MacAllan - Executive Officer (XO) on the IMC flagship IMS Odyssey - would grow disillusioned with the IMC's treatment of the Frontier's citizens. Eventually, he would be given control of the vessel by his Commanding Officer Marcus Graves and told to "take the ship", crashing it on planet Troy. Officially, MacAllan would be branded a traitor, defector and mutineer.
Not much is known about the majority of the time between the Titan Wars and the events portrayed in Titanfall, except for the few months leading up to The Refueling Raid. In these months, Commander-In-Chief, Frontier Command (CINCFRONT) Graves would pursue the 1st Militia Fleet relentlessly, hounding them at every turn and slowly wearing down their fuel and supplies. At some point in this time, the Militia's General Anderson would be killed in action and command of the Marauder Corps (M-COR) handed down to Sarah Briggs.
The 1st Militia Fleet would stage an attack on an IMC refueling facility, dubbed the Fracture Operation. Stealing vital fuel, they would then retreat into Sector Bravo-217 to hide. This retreat would lead the IMC to discover Colony G21, and with it MacAllan and the wreckage of the Odyssey. The following massacre, conducted by Kuben Blisk, would see the initial deployment and testing of the BRD-01 Spectre with many colonists killed or captured and sent to ARES Division for testing. The Militia forces would succesfully extract MacAllan and a handful of survivors, however, leading to the ex-officer joining their cause. Eventually, the Militia would go on the offensive, crippling the flagship IMS Sentinel, destroying the fleet stationed at Airbase Sierra and eventually attacking the port and refinery world of Demeter. By destroying the facility, Militia forces would cut off the IMC's logistics train for many years, stranding the surviving forces on The Frontier indefinitely.
At some point, the Militia would also begin operations on Gridiron, setting up a Special Forces base in an abandoned reservoir, ready to strike at Training Ground Whitehead less than 100 miles away. They would also conduct a prisonbreak on the IMC prisoner waystation codenamed "Overlook", while the IMC would continue to attack Militia-allied settlements such as the Nexus and Smuggler's Cove.
In the wake of the Battle of Demeter, IMC forces reorganised, under the command of Vice Admiral Spyglass, into the "Remnant Fleet" following the defection of Marcus Graves and vast quantities of human personnel to the Militia cause. Some of the Militia's first targets would be the facilities producing the IMC's Spectres, with Graves and Barker providing the locations of numerous facilities such as the Hammond Robotics Corporate HQ and Zone 18 production plant. Nonetheless, these strikes would not completely eliminate the IMC's logistics, with new automatons such as the Stalker, Tick, Reaper and Simulacrum all being pressed into service.
Shortly after the Battle, IMC Expeditionary Force would set up the Fleet Operations Base on a largely-uncharted world, displacing the residents in the process. Almost immediately, the fleet would begin mass excavations, eventually uncovering alien ruins in the swampland surrounding the base. However, the Militia would soon respond to SOS signals sent out by the colonists, leading to one of many battles fought by the now-evenly matched armies.
The Remnant Fleet would soon begin a number of resource raids, targeting allied and enemy systems alike for reasons presently known to no-one but Spyglass himself, soon leading to some IMC units declaring the Remnant Fleet a rogue element. Despite this, resource raids conducted on the planet Harmony, the dig sites surrounding the resort-city Haven, and many other worlds would net the IMC the materiel needed to sustained a protracted war effort. IMC doctrine would ever-more focus on the deployment of robotic infantry, now making up substantial numbers of IMC manpower. In addition to this, mass amounts of prevuslusy-mothballed equipment would be brought back into service such as the Sand Trap fuel reserves.
Despite their resistance, the IMC would slowly begin to lose ground to the Militia war machine. As the initially-expected two-year deadline for the construction of a new gateway at Demeter passed, the fleet was still stranded on the Frontier - and would be for another three at the minimum. In the meantime, the Militia would be transformed from a loosely-organised band of rebels into a professional military, with the organisation, supply chains and everything else that entails. The Militia would begin to manufacture their own war material, such as the Vanguard Titan chassis and MacAllan-Class Carrier, ditching their need to steal supplies from IMC garrisons. In this time, the Militia would begin Operation: Broadsword, a widespread push across the Frontier to further drive the IMC back.
With the Militia having taken around a quarter of the Frontier in the five years following Demeter, the Militia would begin a massive operation to begin a new Frontier-wide push to further assist the war effort. This operation would see many battles, including the Battle of Typhon. The assault on Typhon would initially fail, though would see the near-destruction of mercenary group the Apex Predators and destruction of an IMC superweapon. Following the battle, Jack Cooper would become one of the IMC's most wanted targets in Angel City and Vinson Dynamics would re-engineer and upgrade two salvaged Vanguard-class Titans, later becoming the Monarch.
Conclusion and Aftermath
The final battle of the war, which happened after the "communications blackout of 2715", was fought at Gridiron from which no one was reportedly able to return. It is unknown how much time passed between the Battle of Typhon and the final battle at Gridiron but it's understood that the war ended in the IMC's defeat and the Militia's victory, though both factions would seemingly disappear afterwards and leave the war-ravaged Frontier to govern itself. The people of the Frontier would get to cleaning up or relocating to other planets, including migrating to the Outlands on the fringes of known Frontier space which was also where the blood sport Apex Games started. There may still be pockets of remote fighting and individuals still claiming loyalty to the IMC, but details are sparse.
In the introduction to the game Apex Legends, the text states that the Frontier War went on for "hundreds of years". However, the timeframe provided between the War's start following the Titan Wars and the war's end some time after the Battle of Typhon do not support this statement. It is possible that the text in this introduction was referring to the entire conflict on the Frontier between the IMC and Militia including the Titan Wars, which would line up better with the details known about the conflict.
A couple of new information about the end of the war was also included in a lore tidbit for Apex Legend Bangalore which state that a communications blackout happened on 2715 and the final battle of the war fought at Gridiron, after which "no one returned".