Thousands of years before the Battle of Metropolis, Darkseid, ruler of Apokolips and ruler of the New Gods, commanded his herald and nephew, Steppenwolf, to conquer Earth. Steppenwolf, known for having never lost a battle before during his time conquering other worlds, brought with him a massive army of Parademons and the Unity, the apocalyptic power that can transform worlds to the landscape of Apokolips itself, through the use of three Mother Boxes. Steppenwolf wanted Earth's forces to bend to him before destroying and remaking Earth.
Before this attempted conquest, the Guardians of the Universe foresaw the threat of Steppenwolf, and created the Green Lantern Corps as the universe's police force and attempted to hold Steppenwolf back multiple times, to no avail. Earth also has its own internal issues: the Olympians saw that one of their own, Ares, was disgusted by Zeus' creation of mankind and tried to poison their minds through violence. Seeing this, Zeus created the Amazon race to spread love and kindness to mankind and combat Ares's influence.
Steppenwolf arrived in magnificent fashion, bringing with him untold numbers of parademons, Apokoliptian dropships, and the Mother Boxes, through which he would attempt to recreate Earth to the face of Apokolips. The tribes of man rallied together to fight the parademons and Steppenwolf, however, the New God carved through swathes of humans with incredible ease, and those who died were resurrected as parademons, increasing their numbers through every human death. The humans were clearly outnumbered and outmatched, but held hope that others would join them.
- "We should have died. But we formed our own unity. Amazons, Atlanteans, all the tribes of men fought side by side. Warriors of legend, allies from other worlds, even the gods themselves all acted as one, laying down their lives to drive Steppenwolf away."
- ―Diana to Bruce Wayne[src]
With Earth threatened by Steppenwolf and the power he brings through the Mother Boxes and his Parademons, all forces of Earth formed an alliance to defeat Steppenwolf and his armies. Amazons, who were created by Zeus to protect humanity, sprung to action immediately, led by Hippolyta and Antiope to assist the tribes of Men already fighting. Atlanteans, led by their king, later joined the fight, bringing their fighting prowess together with the Amazons to bring the war to equal footing. Help also arrived from other worlds through the Green Lantern Corps, who sent one of its members to assist Earth's defenders, and from the Olympians themselves, with Zeus personally leading the fight against Steppenwolf, and his fellow Olympians Ares and Artemis joining him.
- "Some day drove him mad. The disgrace of his first retreat. He swore our alliance would crumble. That darkness would cover the Earth. And that he would return when it did."
- ―Diana to Bruce Wayne[src]
With all the forces of Earth united and help from the Green Lantern Corps, the alliance of Earth fought against Steppenwolf and his Parademon armies in one massive battle to defeat Steppenwolf and force him to retreat.
The Amazons, Atlanteans, and Humans charged together against the Parademons, with Amazon general Antiope leading a charge against the Parademons and killing several. Yalan Gur then joined the fray personally, using his powers to clear the skies of Parademons. The Olympians also joined in the fight, with Artemis using her powers as the Goddess of The Hunt and Ares using his skills as the God of War.
Steppenwolf, however, was more than powerful against any opposition and killed Gur. As he was distracted by the power ring that returned to the Corps, Zeus used his lightning powers to attack Steppenwolf and combined with Ares to injure Steppenwolf. Whilst this was happening, the combined alliance of Earth's defenders overwhelmed the Parademon army enough that they were forced to retreat, with Artemis shooting down at least one Apokoliptan dropship.
Forced to retreat for the first ever time, Steppenwolf tried to come back to the fight but was held back by his advisors, and swore that the alliance would crumble, then he would return. Zeus projected a powerful electrical blast to separate the Unity to render the Mother Boxes dormant. This supposedly, drove Steppenwolf to madness, as well as the humiliation of his first defeat ever. With that, the alliance of Earth was victorious.
- "Left behind, the Mother Boxes were too dangerous to be kept together. One was entrusted to the Amazons. One to the Atlanteans. Both sealed and guarded in secure strongholds. The Box of Men was buried in secret, so that no tribe would be tempted to use its power over another."
- ―Diana to Bruce Wayne[src]
With Steppenwolf and his armies held back, the alliance honored their dead and found the 3 Mother Boxes deserted by the Apokoliptian forces. Deeming them too dangerous together, the Mother Boxes were then separated: One was given to the Atlanteans, one to the Amazons and one to the humans. The Mother Boxes given to the Atlanteans and Amazons were sealed and guarded in vaults that was specifically built for holding those Mother Boxes. The box that was given to the humans were then buried by the leaders of the human tribes, who swore to never use it against one another.
The harmony made from the invasion soon past and crumbled: Ares declared war on the other Olympian Gods, killing them all, and nearly killing Zeus, but was then himself injured gravely. Zeus then created Themyscira as a sanctuary for the Amazons and entrusted the God Killer to the Amazons, predicting that Ares would return, but forbidding the Amazons from leaving Themyscira. The Atlanteans were forced undersea when their kingdom of Atlantis was struck by disaster. Mankind soon forgot about the events, leaving them squabbling against themselves.
- Zack Snyder and Weta Digital revealed the fiery design on the ground during the battle scene was originally intended to be Darkseid's Anti-Life Equation. 
- A mural from Zack Snyder's Justice League depicts the whole invasion with Darkseid.