Answer:
Imperialism
Step-by-step explanation:
The most responsible for the huge casualties of World War I is "IMPERIALISM"
This is evident in the two major crises that happened in Morocco in 1905 and 1911.
In 1905, Wilhelm II the German Emperor and King felt he was humiliated despite having his empire in Morocco.
In 1911, France with the help of Britain took over Morocco from Germany and later gave Germany Central Africa.
This issue of imperialism led to Militarism and then Alliances were formed. And the war was fought based on Nationalism.