How the course of events went from being a local conflict to a world war ?

  It helped bring a quick victory to the Allies.

The United States had been providing some support to the Allies prior to its own entry into the war.  But when the US joined the effort fully with a declaration of war, that not only meant the arrival of fresh US troops but a commitment of even more American dollars to the Allies' war effort.  The American commitment greatly boosted the morale of British and French troops on the Western front and demoralized the German troops.

As to other answers I've seen suggested with this question in multiple choice format:

because the earth is a round globe. but that is just what they tell us it is flat and those dudes actually where lying they sailed east.