The first critical decision made in the campaign was a big one. It was the Confederate decision to concentrate at Corinth. As the spring campaign began in 1862 the Confederacy was still of the mindset that they had to defend every point. They had large numbers of men in various ports of the Gulf coast and had variously sized "armies" along the front in Kentucky. The Union shadowed these dispositions in Kentucky but they had more men at each place. For the Confederacy to do something against these forces they would mostly likely need to concentrate and attack before the Union could react. After the fall of Fort Donelson in mid-February Beauregard pushed for such a concentration and was able to have his plan approved. Soon men from New Orleans and Florida came north to Corinth to join the various elements from Kentucky that were also concentrating there. In the days after the fall of Donelson Johnston told Beauregard to go to Columbus, Kentucky, and command the "army" that was west of the Tennessee River while Johnston would command the army that was east of the river. If this dual army situation had continued it probably would have turned out badly for the Confederacy.
The Confederacy basically had two options. They could have concentrated elsewhere or not at all. If they don't concentrate at Corinth the Union has an open road to the strategic rail junction. Corinth was the only place in the Confederacy that a large north-south railroad and a large east-west railroad crossed. Keeping Corinth in the Confederacy would aid their war efforts in the theater by making transport of men and supplies much easier. Corinth would certainly have been the Union’s next goal and thus the Confederates would likely have put a large force here to stop them. If the Confederates hadn't tried to hold Corinth the Union would have likely gone there, destroyed the rails and then focused its attention on the closest Confederate army. What makes it a critical decision is that troops from many areas of the Confederacy were concentrated here. This was a large shift in Confederate policy at the time. Prior to this they were trying to defend everything but after the fall of Fort Donelson they pulled troops out of many areas in hopes of winning a major battle that would restore the lost territory.
The shift in overall strategy makes it a critical decision. Concentrating at Corinth was needed but the way it was done is critical to the course of the campaign. The Confederacy was now gambling on their future by defending key points and not trying to defend everything. Once the shift in strategy is decided upon Corinth is the obvious place to concentrate.