How did Battle of Shiloh show Union and Confederate Generals that the Civil war would not end quickly?

The confederates didn't "stop" the union at the battle of Shiloh. Hard to ether side the victor, but the confederates retreated. The battle of Shiloh showed both sides that a decisive victory capable of ending the war outright wasn't possible. The askers summation is rather accurate. The Confederates would have won if every thing had gone to plan, but it didn't. The battle raged/stalemated on the first day. Reinforcements showed up for the second day, and the union counter attacked. One of the confederate generals catching a stray shot (fatally), and the counter attack forced them to retreat.