The Book Thief is steeped in war. It's set primarily between 1939 and 1943 in Nazi Germany. Both the Holocaust and World War II are going on at this time. Warfare shapes the characters' lives and impact their choices. The narrator of the tale, Death, doesn't take kindly to war. Because of it, he can't get a vacation, or even a slow day. This gets us into some controversial territory. Some would argue that the Allies had to make war on Germany in order to stop the Holocaust and Hitler's plan to take over the world. While Death doesn't suggest an alternative to war in situations such as this, he does raise the questions through his anti-war stance.
When Hans gives bread to a Jewish man, he's performing an act of resistance against the war.
In spite of the civilian tragedies, bombing Germany was necessary to stop the war.
There are always non-violent solutions. The Allied bombings of Germany were not justified.