In the Halo universe, Humans are engaged in a decades-long conflict with the extraterrestrial hegemonic empire known as the Covenant. In addition to a whole host of diverse lifeforms that serve in their armies, their religious fervor makes them an especially dangerous threat to humanity.
This conflict began on a single world but has grown to galactic proportions. The timeline is long and complex. Many details are expanded upon in outside media such as novels or shows rather than in the games. Players may have missed these details over the years due to the decentralized nature of this information.
10 The Seeds of Divine Conflict
While the Halo games drop the player right in the middle of the conflict, the novel Contact Harvest details the very start of the war between Humanity and the Covenant. The Covenant's religion would play a major part in the march to war. They believe themselves to be the heir to the Forerunner legacy.
In reality, it is Humanity who was chosen by the Forerunners to be their heirs. The Prophets, leaders of the Covenant, discovered this fact. They would wage a genocidal holy war in order to retain their power over the empire.
9 The Forerunners and Religion
The origins of the Covenant's religion begin with the activation of the Halo Array 100,000 years before the first game. Halos are weapons designed to kill all organic life in the galaxy in order to starve a parasitic organism known as the Flood. The galaxy was then reseeded by Forerunner A.I. and would once again be saturated with life after the Flood was gone.
To the Covenant, the halos aren't weapons but divine instruments. They believe that activating the rings will start "The Great Journey" and allow them to transcend into godhood like the Forerunners that they believe chose them to be successors. They are mistaken.
8 Three Decades of Warfare
Most of the Games do not give the player any indication of how long the war has been going on. Halo 3: ODST indicates that humanity has lost whole worlds and that they are on the losing end but doesn't give a specific duration to the war.
Hardcore fans will note that the book Contact Harvest takes place in the year 2525. It also shows the opening battles that started the war. The Halo trilogy begins in the year 2552, which means that the war raged for a whopping 27 years. It would finally come to an end with the death of the Prophet of Truth in this same year.
7 Humanity on the Defense
The games show probably the best parts of the war for humanity. In just one year, the humans were able to discover a halo ring and destroy it, which sent reverberations throughout the Covenant. Halo: Reach and ODST, however, show the reality that Humanity faced for most of the war.
Humans were getting slaughtered on every world that the Covenant found. Their worlds would be glassed until they were nothing but a barren wasteland remains. Indeed, Humanity was on the losing side until they were miraculously saved by the Prophet's incompetence of all things. After the war Humanity would lose some 23 billion of its people.
6 Spartan Super Weapons
Despite what many players may think, Spartans were not made to fight the Covenant. Before the UNSC made contact with the Covenant, it was engaged in a conflict with various insurrectionist cells throughout its territory. The Spartan I and II programs were designed to combat this problem, with Spartan II's being the first real success, though only after drastic steps in the form of kidnapping children and running horrible experiments on them were taken.
Only after the Covenant attack did this change. Spartan III's were made specifically to fight the Covenant as mass-produced, expendable super soldiers. Spartan IVs were made after the end of the war.
5 Honor Bound Elites
Elites are a fan favorite faction in the Halo universe. Reminiscent of samurai in their devotion to principles of honor, they have an immense respect for a worthy adversary. As one of the founding member races of the Covenant, they enjoyed a high position inside the leadership structure before the loss of the first Halo ring.
After that, the trust placed in them by the Prophets slowly began to waiver. It culminated in the attempted genocide of their race at the hands of the Brutes, pushing them to join the humans. After the war, they would continue to fight the Brutes out of revenge.
4 The Sacred Halo Array
Halos are a major fixation for the Covenant. They form the backbone of their religion, which promises the discovery of the rings, beginning the so called "Great Journey". Players may have missed the inherent weakness in this. Forerunner installations were discovered towards the end of the war, and many battles took place on their surfaces.
Traditionally, the Covenant chooses to use their massive ships to glass the surface of human worlds in order to win battles. On Halo rings (and the Ark), they wouldn't dream of damaging one of their "sacred rings". The humans were free to wage whatever kind of conflict they wanted. The Covenant on the other hand were unable to use their greatest weapon to stop them.
3 The Flood And Their Origins
This ancient parasitic hivemind is quite possibly the scariest thing in the Halo universe. They were created by an ancient alien race known as the Precursors, a race even older than the Forerunners, though their relationship was sour. The former was destroyed by the latter, and out of revenge, the Precursors unleashed the Flood upon the galaxy.
They would go on to bring the Forerunners to ruin and even play a major role as a wildcard during the Human-Covenant War when they returned 100,000 years later. This origin was never revealed during the games, but the novels explained it in great detail.
2 The Banished: Space Pirates
The Banished are basically the Covenant but without the religious baggage. Led by the Brute, Atriox they are a group of ex-Covenant criminals who steal advanced technology, which they then sell to the highest bidder. They were formed during the Human-Covenant War and became a minor thorn in the latter's side.
It was during the Battle of Algolis that Atriox rebelled against his Elite Masters. He formed the Banished along with his Brute comrades. After the war, they would grow to become successors to the Covenant in many ways.
1 Humans And Forerunner Technology
Often called Reclaimers by Forerunner A.I, humans have always had a strange relationship with their technology. Non-humans like the Covenant cannot actually use forerunner tech and that includes activating the rings. This is because Humanity was chosen by the forerunners to succeed them after their downfall.
Even though the two species were bitter rivals before the firing of the rings, the Librarian saw fit to bless them with the genes necessary to use their technology and eventually take on the "Mantle of Responsibility". The formerly rogue Forerunner A.I. known as Mendicant Bias would assist the Master Chief in secret while he battled the Covenant and flood on the Ark and partially completed Halo ring.