In All Quiet on Western Front, what detail in Chapter 11 indicates the disintegration of the German army?
I believe the detail for which you are looking is expressed by the quote,
"we are falling back, we will not be able to atack again after this big offensive, we have no more men and no more ammunition."
It is the summer of 1918, and the war has been going on for years. The population of young men eligible for military service has been decimated by the number of casualties that have been suffered over that time. Paul describes the lengths to which staff sergeants will go to declare a man fit for duty, recalling the ludicrous story of "a fellow with a wooden leg" who is declared A1. The story has circulated widely among the men in the ranks, and although it indicates the amount of corruption among the military establishment, it also bears testimony to the extreme difficulty Germany is having at this point to provide fodder for its seemingly endless war. The men in the trenches feel this pressure acutely; they are sent back to the front time after time after time. When Kat is injured, his leg bone shattered, he asks Paul bitterly, "Do you think that I will be marked A1 again with this leg?"
There are many other details in the chapter that indicate that the German army is disintegrating as well. Paul says,
..."we are emaciated and starved...dysentery dissolves our bowels...Our artillery is fired out...we have too few horses. Our fresh troops are anaemic boys in need of rest, who cannot carry a pack, but merely know how to die. By thousands. They understand nothing about warfare, they simply go on and let themselves be shot down."
The lack of food, dearth of ammunition and fresh equipment, and the inexperience of the novice troops who are too young and improperly trained, if trained at all, makes it clear to the more experienced remnant of the army that the end is near. Kat says, wonderingly,
"Germany ought to be empty soon" (Chapter 11).