It all depends who's side you're on. Serbia and Russia had an alliance, then France had one with them. That caused Germany to declare war of France due to its alliance Austria-Hungry. To avoid the French MLR Germany went through Belgium, who had an Alliance with England which dragged the entire commonwealth into the war. I'm not sure why the Ottomans and Bulgaria joined the Central powers and I assume Romania was with Russia at this time. America joined in due to the German U-boats attack it's ships. Finally, Italy joied in to get territory but got it's ass kicked.