Prologue/Timeline

Author's Note: The reviewer AznMagicMan has brought up an excellent point on how the Imperium would get extremely pissed off at various aspects of UNSC society. I've actually came with background information beforehand that would ensure the IoM wouldn't try to kill the UNSC right away, but I forgot to put it up when I first put up the story. So…here's a rough timeline of events that led up to the story. Also, I'm assuming that Retribution takes place in early M42 and Space Marine takes place 50 years after that.

Timeline – 40k side

000.M42

A techpriest discovers an STC fragment that appeared to be an add-on for the Golden Throne in the depths of the Martian caverns.

009.M42

A Tyranid hive-fleet attacks subsector Aurelia but is destroyed thanks to the leadership of Force Commander Aramus. In the Cadian sector, Imperial troops are slowly gaining ground against Abaddon the Despoiler's ground troops on Cadia.

010.M42

Aramus' strike force fends off a Black Legion assault on subsector Aurelia. Avitus is discovered to be the traitor and is slain. After Ulkair's banishment, Kyras declares Aramus a renegade and forbids any mention of his name among the Blood Ravens.

015.M42

Five years after Ulkair's banishment, war rages across sub-sector Aurelia. In the midst of the chaos, a small contingent of Blood Ravens still uncorrupted by Kyras consisted of Aramus, Thule, Thaddeus, and twenty other Astartes are commanded by Gabriel Angelos to evacuate Governor Derosa to an unknown location. Abaddon now controls less than 10% of Cadia's surface.

020.M42

With a combined force of loyal Blood Ravens, Imperial Guard reinforcements, and an Eldar warhost, Azariah Kyras is defeated on Cyrene, while Ork freebooters hired by the Inquisition wreak havoc across the subsector in order to distract Kyras' forces. Eliphas is resurrected by Abaddon to both eliminate Kyras as a potential rival as well as to wreak further havoc in hopes of distracting the Imperium from Cadia. However, Eliphas is killed for the third time by Apollo Diomedes. A sorcerer in Abaddon's retinue named Nemeroth, dissatisfied with Abaddon's handling of the Black Crusade, convinces some subordinates to munity and forms his own warband.

Gabriel Angelos becomes Chapter Master, and Derosa returns from hiding along with her Blood Raven guards. Chapter Master Angelos enacts numerous reforms, including permanently splitting the offices of Chapter Master and Chief Librarian, the destruction of all Chaos-tainted relics, and more stringent testing for Librarian candidates. Aramus, Diomedes, and Tarkus are rewarded with captaincies for 1st, 3rd, and 4th companies respectively. Thaddeus becomes a vanguard sergeant. Anteas, who accompanied Thule in the Kronus Campaign, becomes Chief Librarian.

032.M42

Cato Sicarius is mortally wounded in battle against the forces of Chaos. Sergeant Trajan, a relative of the 2nd Company Chaplain, is promoted to Captain in his stead. Sternguard Sergeants Titus and Sidonus are transferred to 2nd company.

050.M42

A Mechanicus explorator fleet discovers a desert planet, tentatively designated to be LVS-1138, in the Ultima Segmentum with ruins that do not match any Imperial databases. Ordo Xenos also fails to identify the origin of the ruins.

060.M42

Captain Trajan is killed in battle against the Eldar. Titus becomes Captain of 2nd company.

062.M42

A Deathwatch Kill-Team led by a radical Ordo Xenos Inquisitor discovers what appears to be a portal, big enough to barely fit a Land Speeder, deep within the ruins on LVS-1138. Inquisitorial presence on the planet is boosted in order to further study the ruins and the portal.

070.M42

A token Ultramarine force consisting of Captain Titus, Sergeant Sidonus, Battle-Brother Leandros, and two Thunderhawks arrive on Graia to liberate the planet from Orks. Titus repels both Ork invaders and a Chaos incursion orchestrated by Nemeroth using a daemon possessing a deceased Inquisitor. The Imperium is able to retake Graia with the help of a liberation fleet as well as Astartes from both the Blood Ravens and Black Templars chapters, but Sidonus and countless guardsmen lose their lives in the process. Captain Titus is investigated due to Leandros' allegations of Chaotic taint but is let go after three days. Leandros is censured. The battle of Graia is noted to be the Blood Ravens' first campaign outside subsector Aurelia since Gabriel Angelos' ascension to Chapter Master.

071.M42

The STC is discovered to be several different upgrades for the Golden Throne that was somehow neglected during the Horus Heresy. The Golden Throne is swiftly upgraded in the hopes that it would preserve the Emperor's life. It is discovered that the STC hosts a keyboard as well as a machine spirit dictates the Emperor's typed words into sound. Galaxy-wide celebrations occur. The LVS-1138 portal, which failed to activate despite numerous attempts, suddenly powers up. Four figures carrying autoguns and an unknown make of power armor come out of the portal. Upon detention and questioning, they reveal themselves to be soldiers from a human civilization in a parallel universe. The discovery creates uproars in the upper echelons of the Imperial government. LVS-1138 is locked down. The Emperor, through the Golden Throne's newly found archaotech device, orders the High Lords of Terra to not undertake any Crusades against the human civilization on the other side and forbids any kind of "intervention". The discovery of the interdimensional portal is designated Top Secret and is known only to the High Lords, the upper echelons of the Inquisition, and the Emperor Himself. A Deathwatch Watch Fortress is built over the unidentified ruins to further guard this secret.

Timeline – Halo

2558

[CENSORED] Spartan-117 [REDACTED] pursuit by Spartan Locke [DATA EXPUNGED] Arbiter Thel 'Vadam [REDACTED] Admiral Osman [DATA EXPUNGED].

2559

During a survey mission, an ONI science vessel, escorted by UNSC Infinity, discovers Forerunner ruins on a previously unknown planet on the edge of UNSC space. Survey teams on the surface discover what appears to be a portal that could fit a Warthog. After activating the portal, Fireteam Crimson is sent through to investigate. They return with news of a parallel universe with a galaxy-spanning human empire on the other side. The discovery is classified and is known only to the upper ranks of the UNSC and UEG. ONI establishes a base on the planet to further study the portal. An envoy emerges from the portal and identifies herself as an Inquisitor and a representative of the Imperium of Man. This marks the first of the many "diplomatic visits" between ONI and Inquisitorial representatives.

Combined

2559-2561/071.M42-073.M42

The information exchanged over a period of two years allows the UNSC and the Imperium to further learn about each other's history and civilizations. Although the Mechanicus and Ecceliarchy are both livid about a human civilization that is both technologically innovative and outside The Emperor's light, The Emperor refuses to retract the order to leave the UNSC alone. Any complaints about the UNSC being allied with former Covenant xenos are made moot by The Emperor's argument that the xenos chose to make peace not because of humanity's weakness, but because of humanity's courage and determination. Ultima Segmentum Lord Commander Vance Stubbs further illustrates the ridiculousness of an Imperial crusade to the UNSC by pointing out that the portal cannot even fit a Valkyrie, let alone a fleet.

On the UNSC side, the UEG is horrified by the Imperium's religious fanaticism which reminded them of the Covenant. However, reassurances from Imperial envoys regarding The Emperor's policy of non-intervention convince the UEG that an alliance could be made with the Imperium of Man. Over a period of two years, representatives from both sides of the portal work out the details to the first exchange between the UNSC and the Imperium of Man, where two Spartans and two Astartes will cross into each other's realms to facilitate cooperation and mutual knowledge sharing.