Why are Paul and his comrades sent to the front?

As Erich Maria Remarque's All Quiet on the Western Front opens, narrator Paul Bäumer and his comrades are resting behind the front lines. Of the 150 men in their unit, only 80 had survived their last trip to the front. In the bitter irony that characterizes the warfare presented in All Quiet on the Western Front, the men rejoice that they get double-rations (this, of course, was only possible because nearly half the men in the unit had died). Paul narrates many other ways in which warfare had already changed him and his friends (coveting the clothing of a dying man, viewing a latrine as a luxury, etc.).

Unfortunately, Paul and his comrades are sent back to the front. Their mission is dangerous; they are tasked with laying barbed wire fence. They face constant shelling and gunfire as they crawl from foxhole to foxhole to fulfill their orders.