How did nationalism and alliances led to ww1?

Serbian terrorist group called the Black Hand killed Arch Duke Ferdinand, from Austria Hungary. Serbia had and agreement that if Austria Hungary invaded them, they had Serbia back, eventually Austria Hungary invaded Serbia, in which Russia invaded Russia, but Germany invaded Russia because it had an alliance with Austria Hungary and if Germany ever invaded Russia then France would attack Germany and Austria Hungary, but German invaded France through Belgium, who was neutral in all this, and that made Great Britain angry and they declared war on Germany and Austria Hungary, overall, lots of alliances led lots of countries to go to war against each other