Answer:
They had pledged to fight with other countries.
Step-by-step explanation:
After the death of archduke Ferdinand in 1914 bunches of nations immediately joined World War I since they had officially swore to battle with nations according to their own allegiances. The territory for war was basically set, and unions were at that point made, the interests were known and on the table for couple of years before that, so the death of archduke Ferdinand was only the point that denoted the beginning stage of the practically unavoidable war.