I'm trying to teach myself about WWI and, when learning about complex historical events, I find it useful to orient myself with the important dates as like mile markers. However, I can't find anything about Germany declaring war on Serbia. I see Austria-Hungary declaring war, then Russia mobilizing in defense of Serbia, and then Germany declaring war on Russia and then on France. So did Germany ever formally declare or was it like an "an ally of my enemy is my enemy" kind of thing?