parsifal
Colonel
The reasons for WWI are complex, and indeed there is no universally acknowledged theory as to who was responsible.
Determining who was the aggressor nation or alliance is easier. It started with a political assassination, saw the victim (Austria) begin a pogrom on a nation allied to Russia, which then saw mobilzatiuon of Russia, france, germany and finally britain (to protect Belgium). The actual first shots and agressive acts were carried out by Germany and Austria. Therein lies your two nations responsible for the outbreak of the war. Other countries are guilty of posturing, brinkmanship and poor judgement. These two nations were the two clowns that actually started the shooting first (along with, perhaps, the nation of Serbia).
Determining who was the aggressor nation or alliance is easier. It started with a political assassination, saw the victim (Austria) begin a pogrom on a nation allied to Russia, which then saw mobilzatiuon of Russia, france, germany and finally britain (to protect Belgium). The actual first shots and agressive acts were carried out by Germany and Austria. Therein lies your two nations responsible for the outbreak of the war. Other countries are guilty of posturing, brinkmanship and poor judgement. These two nations were the two clowns that actually started the shooting first (along with, perhaps, the nation of Serbia).