The Daxamite Invasion was the hostile attack in National City, Earth-38 by the alien race known as the Daxamites who had the goal of conquer the planet Earth and make it the new home for the Daxamites as well as sate Rhea's vengeance against Supergirl for turning her son against her.
However, the invasion was prevented by Lena Luthor, who spread lead into the Earth's atmosphere, causing the Daxamites to retreat.
Rhea's son Mon-El didn't want her to destroy National City and was forced to join Rhea's team.
In addition to the loss of life, alien and human, as well as left over technology for criminals, both aliens and humans, to use, it also left lasting impressions throughout the nation. It led to an anti-alien movement that Lex Luthor took full advantage of in order to become a hero of the United States.