What is "the abyss" to which Paul fears his thoughts will lead in All Quiet on the Western Front?As Paul stands guard over the Russian prisoners,...

The "abyss" to which Paul fears his thoughts will lead is the conclusion that the war which has destroyed his life and the lives of all his friends is based on a fallacy, in which case the sacrifices of his generation will have been for naught.

Paul and his classmates had been pushed to enlist by their teacher and other older men of that same generation.  They had been told that they are the "Iron Youth", that war is glorious, and that they have a duty to fight for their illustrious country.  Not knowing any better, the boys have given in to the pressure and been sent to the front lines in the German army.  Once there, Paul and his friends discover that they had no idea about the true horror of war.  One by one their lives are destroyed, and those who survive are irrevocably changed.  A whole generation is rendered rootless and without hope, unable to ever to return to their lives as they remember them.

To make things worse, as Paul performs his duty of guarding the Russian prisoners, he is aghast at how much they are like him and the people he has known.  He sees that

"they have faces that make one think - honest peasant faces...they ought to be put to threshing, reaping, and apple picking.  They look just as kindly as our own peasants in Friesland". 

Once Paul begins to see the enemy as individuals sharing the same humanity, he starts to question the very basis of the situation which requires them to face off as hated opponents and kill each other.  He thinks,

"A word of command has made these silent figures our enemies; a word of command might transform them into our friends.  At some table a document is signed by some persons whom none of us knows, and then for years together that very crime on which formerly the world's condemnation and severest penalty fall, becomes our highest aim.  But who can draw such a distinction when he looks at these quiet men with their childlike faces and apostles' beards...we would shoot at them again and they at us if they were free".

Paul's thoughts frighten him because he has begun to question the very nature of war in all its absurdity.  Deep down he realizes that such thinking can lead only to the conclusion that war is senseless, and the waste of his life and that of his generation has been for nothing" (Chapter 8).