KANSAS CITY, Mo. (AP) — The impenetrable wall that was the Kansas City defense was breached by the Denver Broncos two weeks ago, when Peyton Manning and his cohorts ran up and down the field in a 27-17 victory.It crumbled entirely against Philip Rivers and the San Diego Chargers last weekend.Now, what was once considered among the most fearsome groups in the NFL is a wounded one — both physically and mentally — as the Broncos prepare to return to Arrowhead Stadium on Sunday.The Chiefs had allowed just a shade over 12 points a game during a 9-0 start, and were piling up sacks at such a rate that they were on pace to shatter the league record. But after a 41-38 loss to the Chargers that same bunch has now allowed an average of 34 points in their two defeats.Things like missed tackles and blown assignments, especially in the passing game. Manning threw for 323 yards and a touchdown without an interception at Mile High Stadium, and his offensive line never let the Chiefs lay a hand on him.