The Nazi invasion was an invasion on Earth-1 organized by the Nazis of 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.