Many factors can be cited as reason for the failure of Operation Barbarossa but pausing at Smolensk and redirecting forces was a crucial mistake by Hitler.
Not totally, or primarily Hitler’s fault. He set three objectives for 1941: Destruction of the Red Army in the area comprising eastern Poland,and the western reaches of the USSR, Leningrad and the Baltic States, and Ukraine. oscow wasn’t one of them. That’s Halder’s baby. And Halder deployed the bulk of the panzers for that purpose.
Result? By the time the Germans reach Smolensk, Bock’s rigt flank is in the air, and he’s far in front of Rundstedt, who, underarmored, and facing the largest [and best equipped Soviet Front, is agging way behind.
Hitler went to Smolensk to do what his generals wouldn’t or couldn’t; make a decision. Opposed to diverting Guderian’s panzers south were Brauchitsch, Halder, Bock and Guderian. Favoring the diversion were Rundstedt, and Kluge and Strauss, the commanders of Bock’s infantry armies, and the generals faced with [among their other tasks] protecting that right flank.
Non-diversion would allow AG Center to continue an early drive on Moscow, with an ever-extending right flank. Diversion would allow the Germans to shorten their lines, freeing up infantry troops for other tasks, acquire one of Hitler’s economic objectives, Ukraine, and remove a sizable portion of Stalin’s Order of Battle off the board.
So it’s not all Hitler, and it’s not as foolish as it may initially look.