The Earth-X invasion was an invasion on Earth-1 organized by the New Reich from Earth-X. They were looking for Supergirl so that they could steal her heart for her Earth-X doppelgänger. However, the heroes of Earth-1 along with Kara, and her adoptive sister Alex Danvers, managed to prevent the attack.
On Earth-X, the Nazis won World War II and spread chaos around the world. After the war ended, the New Reich was formed by Führer Hitler. After Hitler's death, a new Führer was put in his place and created the New Reichsmen, a group of elite Nazis that were commanded by Overgirl.
On account of absorbing too much solar energy, Overgirl's body was overloaded, and by making excessive use of her powers, her heart would ultimately fail. Shortly after discovering this, the Reverse-Flash of Earth-1 arrived and joined the New Reichsmen. After much research, they managed to find a new heart for Overgirl, and it was from her doppelgänger of Earth-38, who happened to be travelling to Earth-1 for a special occasion.
Before the Invasion
At some point before November 2017, the New Reich had meet with Eobard Thawne, and they became ally. At some point, after it was discovered that Kara/Overgirl was dying, they came to learn about a doppelgänger of Kara that could save her life if they took his heart.
When an opportunity as the form of a wedding happened with the presence of Kara Danvers, the New Reich launched an attack without warning Thawne. The attack failed causing the capture of Tommy Merlyn/Prometheus and several of their soldiers to die.
They then attack Dayton Optical Systems and stole a device called The Prism, where they were confronted by Supergirl, Green Arrow and The Flash. The three leader of the future invasion removed their masks to reveal to the heroes just who they were, which shocked them. A fight happened, and they managed to escape with the Prism.
A day later, with the Wellenreiter operational, and the Heroes of Earth-1 captured on their earth, they tried to revove Kara doppelgänger's heart but failed due to the intervention of the Legends. They then proceeded to plan B which was invading Earth-1. The soldiers started to kill civilian but the heroes of Earth-1 made it to Central City and started battling the Earth-X soldiers. The heroes emerged victorious since Overgirl exploded, Dark Arrow was killed and Eobard Thawne abandoned them to their demise.
In the aftermath of the invasion, the world isn't aware of the multiverse or that the fact that those Nazi invader were from a different universe. One of the most significant impacts was that Killer Frost was now considered a hero rather than a villain.