"Erika" is a German millitary marching song. It's also mostly popular among the type of people who love the aestethics of German millitarism. It's not terribly different from some of the other "I've got a girl at home and I must fight for my nation to protect her" type of millitary songs, aside from the listeners being way further right. Like, I'm pretty sure last time I checked that half the comments on the song on youtube used to contain either blatant anti-semitism or other references to the Nazis.