Why was the Battle of Shiloh fought?

Grant's Union Army was sent by Gen. Halleck to seize the imp0ortant position of Pittsburgh Landing, where they had to be joined by the Army of Ohio under General Buell. Then a successive advance towards Corinth should have been followed.

At the same time the Confederate Army of Mississippi under Gen. Albert Sydney Johnston gathered at Corinth and started to advance towards Pittsburg Landing, aiming to scatter or destroy the enemy army.

The advance was carried on in a way that it was not possible for the Union outposts to realize in time that a great attack against their encamped units was about to be broken out.