Russia then declared war on A-H, Germany then on Russia because russia declared war on A-H, then France declared war on germany for declaring war on russia, then Germany tried the schlieffen plan and declared war on Belgium and because belgium was a neutral country the british empire declared war on germany.
In short, the alliances (Triple Entente (Russia, France British Empire)and Centralpowers alliance (Germany,Italy and A-H))caused these countrys to declare war on eachother.
