Direct cause of WW1 was continental expansionism by Germany, Russia and Austria. Austria hated the Serbs and was just looking for a war with them to eliminate their pan-slav nationalistic threat, Russia, who had interests in the Balkans (with eyes always on Constantinople) and was 'protector of the Slavs' was not going to allow Austria to absorb another Slavic state. Germany held all the cards - they could have told Austria to back down. But Germany felt it was their time to bring Europe to its knees - quick victory over France (who was sworn to come to Russia's aid) would allow them to then turn against the slow-mobilizing Russians and deal them a lethal blow. Germany did not WANT war with England, they felt threatened by the English fleet, but were otherwise amicable towards them. Germany decided to attack thru Belgium to circumvent the French fortifications along the Alsace-Lorraine border to get to Paris faster and end that front quickly. Unfortunately, the British were sworn to uphold Belgian soverignty and thus England declared war on Germany - Germany didnt think England would get involved in a continental war over a "scrap of paper" but they were wrong, and the two things that lost the war for Germany:
1. Tannenburg and the Masurian Lakes - the French urged the Russians to attack the Germans rear even though the Russians were far from ready - but in doing so (and getting slaughtered) the Germans had to take troops away from their encirclement of Paris to defend against Russia - this made them fail at their atempt at ending the war quickly - only THEN did the war become a stalemate.
2. The british blocakde - Germany fought dor 4 years but only capitulated because they ran out of food and war materiel.
Why WW2 started: two reasons
1. an American wrote the Peace treaty of Versailles - Woodrow Wilson. The treaty should have been written by Europeans - the American enteed the war - basically did little and lost relatively few men, yet THEY write the peace treaty? Not a good idea.
2. because the treaty was so harsh COUPLED with the fact thet Germnay was NEVER CONQUERED, made the Germans feel they got shafterd and they laud in wait 20 years until someone showed them how to get revenge.
WW1 was very interesting at the beginning, but that lasted only one month before the stalemate took away its plans. Read Barbara Tuchman's "The Guns of August" for more on the amazing build-up and first month of The Great War - you will not be disappointed.