It is hard to prove from the novel that there is nothing worse than war, since this would require that you discuss all the other things that are bad in life, but that is a point about the way your essay topic is worded. If it is the topic assigned to you, then so be it. If, however, that is your wording, you might revise it a bit to make it less open-ended. A better thesis to support with the novel would be that war is horrific and pointless.
In terms of three main points to examine in discussing how Hemingway depicts war in the novel, consider these:
- War causes horrible suffering and death.
- War creates chaos and destruction.
- War never ends.
In terms of suffering and death, research Frederic's wounding in battle and his trip to the hospital; notice Hemingway's treatment of Passini's death and Frederic's attempts to save him. The chaos of war is vividly described in the retreat from Caporetto. The idea that war never ends is developed in several conversations Frederic has with others about the nature of war. Good luck with your essay!