The Nazi Invasion was an invasion organized by the Nazis of Earth-X, an alternative Earth where they won World War II. They were looking for Supergirl so they could steal her heart and 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 is commanded by Overgirl.
On account of absorbing too much solar energy, Overgirl's body was overloaded and that by making much use and effort of her powers, her heart would fail. Not long after, the Reverse-Flash of Earth-1 arrives and joins the New Reichsmen. After much research, they managed to find a new heart for Overgirl, and it was from her doppelgänger of Earth-38 that she was going to Earth-1 for a special occasion.