explain the signifacne of American entry to World war 1?

Question

Cause and effects..

Answer 1

The cause was the realisation that the US. could not maintain their previous policy that they ''too proud to fight'' and their interests were interlinked with those of the Allies (Great Brittain in particular) ,realisation which became more evident with the discovery of the Zimmerman note which represented a awkward yet potentialy lethal effort from the part of Germany to intervene in the Western Hemispere and to create trouble to US. by formenting Mexican hopes for revenge against the former.
The effects of the American entry were momentous; without the encouragement given by the mere presence of the US. troops in the Western front as well by their fighing perfomance, the Allies would certainly have lost the Word War I in 1918

Answer 2

The beginning of butting into other people's business. Very costly and now having become a habit.