What roles did the assassination of francis ferdinand and the existence of prior military plans play in leading quickly to the outbreak of world war i?

The military plans laid before World War I presupposed a major war between the countries which were tied together with alliances.  Because the Triple Entente had Britain, France and Russia as allies, Germany thought if a war began it would need to fight on two fronts -- west and east.  So German Field Marshall Alfred von Schlieffen drew up war plans that said attack France first, quickly, and then hold that territory while deploying forces to contend with Russia in the east.   

When the Austrian Archduke Franz Ferdinand was assassinated in 1914, Austria threatened Serbia with war.  Russia supported its junior ally, Serbia, and began to mobilize troops.  Germany demanded Russia stop its troop mobilization, but the Russian military was ready and eager to go to battle.  So declared war on Russia, and in doing so, the first thing it did was to go and attack France.  Thus the war spread and became instantly a more global conflict.  All the military preparations and battle planning made the outbreak of the war like a series of dominoes falling, as the countries of Europe fell into war with one another.