The Archduke of Austria was assassinated by Serbian assassins in Sarajevo, capital of Bosnia and Herzegovina. Austria and Hungary saw this as an attack by serbia and his assassination got the ball rolling, so to speak.
After the assassination of Archduke Ferdinand, various European countries were allied to countries on one or both sides. This spiraled out of control, as, allies had allies and allies' allies had allies. This pulled practically everyone in Europe into the war on one side or the other.
Nationalism in industrialized countries encouraged nations to believe that they were superior to their neighbors and was a large contributor the war, in that everyone thought their cause was just since they though their country was better
The quickly growing manufacturing capabilities of many countries meant that weapons, technology, and all manner fo things could be quickly and cheaply mass produced. This led to the ability to have many more tanks, etc. and much better armed soldiers.
Countries, with their newfound industrial power, and their nationalist pride, set out to acquire other countries' goods and land for their own and to bring their people to their views and ideology. This sort of added on to tangled alliances and added places where an imperialistic nation could have military bases or attack from.
With the capability to mass produce all sorts of weapons technology, industrialized countries did just that. There was a burst in creation of weapons and arms for soldiers so that each country could be the strongest and live up to its nationalist expectations.