What was Lincoln's letter to Union Major General John A. McClernand about?

Hello!

In the letter, Lincoln referred to rumors that Joseph H. had called a dictator to place the Union's highly distorted civil-military relations in the months preceding Joseph H.'s rise to command on a more appropriate basis. Lincoln also decided to address a significant problem that had plagued the Potomac Army during and after George B. McClellan's time in command.

Also this letter has been proclaimed one of "the most eloquent letters" from Lincoln, wrote in a tone of "a wise, benevolent father" while gently chastising Joseph H.