Grant's main army was located at Pittsburg Landing, while 40,000 Confederate troops converged at Corinth, Mississippi.[146] Brigadier General William Tecumseh Sherman assured Grant that his green troops were ready for an attack. Grant agreed and wired Halleck with their assessment.[147] Grant wanted to attack the Confederates at Corinth, but Halleck ordered him not to attack until Major General Don Carlos Buell arrived with his division of 25,000.[148] Meanwhile, Grant prepared for an attack on the Confederate army of roughly equal strength. Instead of preparing defensive fortifications between the Tennessee River and Owl Creek,[p] and clearing fields of fire, they spent most of their time drilling the largely inexperienced troops while Sherman dismissed reports of nearby Confederates.[149]
Union inaction created the opportunity for the Confederates to attack first before Buell arrived.[150] On the morning of April 6, 1862, Grant's troops were taken by surprise when the Confederates, led by Generals Albert Sidney Johnston and P. G. T. Beauregard, struck first "like an Alpine avalanche" near Shiloh church, attacking five divisions of Grant's army and forcing a confused retreat toward the Tennessee River.[151] Johnston was killed and command fell upon Beauregard.