Europe was one big keg of gunpowder waiting for someone to throw a match. The excuse came with the assassinations in Sarajevo.
Austria-Hungary declared war on Serbia, then Russia declared war on Austria-Hungary, then Germany declared war on Russia, then France started mobilizing, then Germany invaded France - via Belgium, causing the United Kingdom to declare war. Things went kinda downhill from there.
Germany was far from innocent but there was plenty of blame to go around.