I am not telling God anything - He is telling us. If God knows everything, and I assume you agree with that, right? -- then how can God "react" to something? I disagree that God changes course in response to prayer, because how can he change course if he knows about the prayer in advance? Then that is not changing course.
......then how can God "react" to something?.....
Please read the account of David in 1Sam 23:10ff.
There one will find David asking God if Saul will come down on him and also asking if the men of Keilah will help Saul.
God's answer is yes, Saul will come down and the men of Keilah will help him.
But did it happen, no, David survived.
Was the whole outcome influenced by the action of men? yes
Truly God knows everything; but that does not limit God to only knowing what is going to happen, for God also knows what would have happened if man had taken a different course.
But through it all, God is so great He can still get done what He wants done in spite of the choices and actions of man.
I like to think of it this way:
You are in a chess match with God. Whom do you think will win???? God no doubt. Why?
Perhaps it is because God not only knows each move you will make in advance; but He also knows each move you could have made.
Therefore it matters not what move is made, God will always know how to counter the move to obtain the end result of His desire.
But all the while in the chess match, God does not instruct or control the choice of the move of the man.
God is much greater than a ruler who has His will carried out simply because He ruled it to be so.
God's plan will come about no matter what choice or direction is taken by man.