Respuesta :

Events at the Marne signaled the demise of Germany's aggressive two-front war strategy, known as the Schlieffen Plan; they also marked the end of the general belief, held on both sides of the line, that the conflict that broke out in the summer of 1914 would be a short one.

So #3. Your welcome. Have a nice day!

The second answer would be the correct one I believe. I apologize if I am wrong.