Humanity was busy fighting itself when they made first contact with the Covenant at Harvest on February 11th, 2525. At first, they were hopeful that they could make friends with the alliance of alien races and grow the human civilization even further than they already had. Their hopes were quickly dashed, however. Attempts to give a peace offering were met with gunfire and death. With almost no military presence so far away from the heart of human-controlled space, Harvest was quickly wiped out, with only a handful of survivors making it off-world.
When contact was lost with the colony, the UNSC and Colonial Military Administration sent four ships to investigate, all of which were almost immediately disposed of. The only contact the ships were able to receive and transmit to the UNSC was the following message:
Your destruction is the will of the gods, and we are their instrument.
What followed was nearly three decades of near constant war. The Covenant made short work of most of the outer colonies before the UNSC was able to muster any sort of successful defense, and while the UNSC did manage several victories, they were never able to turn the tide enough to push into Covenant space. Slowly, but surely, humanity was losing.
FROM BAD TO WORSE
By 2552, humanity's death toll was in the billions. The mass construction of warships, combined with superior tactics, meant the UNSC was able to muster a formidable defense at any colony world left standing. In July 2552, a defense force of nearly fifty warships managed to hold off a Covenant assault of twenty-four ships at Sigma Octanus IV. Winning two-to-one odds was an impressive feat, and the fleet commanders were rightly commended.
Not a month later, the Covenant discovered and invaded the largest inner colony world, Reach. The UNSC threw every military asset at the planet to help protect it. Despite the heroic actions of the men and women of the UNSC, however, the Covenant overwhelmed the colony world with over 300 warships and countless ground troops. Aside from losing one of the major military strongholds humanity had, the battle claimed over 130 UNSC warships and around 700 million lives.
In the aftermath of the Fall of Reach, nearly all remaining UNSC forces were recalled to the defense of Earth. Even so, when the Covenant finally arrived in the Sol system on October 20th, 2552, the UNSC struggled to put up a formidable defense. Through the actions of Spartan John-117, however, humanity was managing to keep up the fight weeks into the battle. Large swaths of the planet were damaged or destroyed, with billions of residence fleeing off-world to try and survive humanity's last stand.
A CHANGE OF FORTUNE
On November 3rd, 2552, the tide of the battle suddenly turned when one of the Covenant leaders, the Prophet of Truth went forward with his plan to replace the Sangheili with the Jiralhanae as the commanders of the military. This sparked a civil war between the Sangheili and the Jiralhanae, known as the Great Schism. With the Covenant forces suddenly turning their weapons on each other, humanity was able to reclaim the upper hand and begin to push Covenant forces off-planet.
The Prophet of Truth made a last-ditch effort to win by using the Portal at Voi to travel to the Ark and activate the Halo Array throughout the galaxy, killing all sentient life. Spartan John-117, along with Arbiter Thel 'Vadam, Commander Miranda Keyes, and Sergeant Avery Johnson followed the Prophet's remaining forces and stopped the Prophet, killing him in the process. The fighting was finally over. Humanity hadn't won so much as the Covenant had lost. But the war was over nonetheless.