from what we can tell the grand army was created on behalf of the jedi council by a jedi called sifadias,but the jedi in question was dead so my idea would be that the emperor(who already planned what would happen with the chancellor ,and the trabe federation,dooku,etc),so there is a possiblility that he had the army created so he would have a stranglehold over the rebublic when the time came to reveal himself
that isnt probably what is going to happen in ep 3 but that is what i think might happen going of what happened in episodes 1.2.4.5.6