Consider The Things They Carried and other war literature. It can be said that the most effective war literature focuses on horror, not heroism. To what extent to you agree?

Great question! This is an opinion prompt, so I can't answer it for you. I can, however, pitch a few ideas to help you organize your response.
The first task I would encourage you to complete in your response is to define "effective." This word indicates to me that the prompt assumes some defined purpose for writing about violent conflict. War literature is a vast corpus of writing that includes everything from racist propaganda poetry to gritty anti-war novels. Both genres had dramatically different purposes when they were published; both could be considered "effective," depending on their historical aim. Clearly outlining what "effective" means in your response will be helpful both for your audience and for you as you structure your thoughts.
Most memorable war-literature, at least from the past few centuries, has explored the horrors of war rather than glorifying it. Stephen Crane's The Red Badge of Courage, Erich Maria Remarque’s All Quiet on the Western Front, the poetry of Wilfred Owen, Johnny Got His Gun by Dalton Trumbo and The Things They Carried by Tim O'Brien all highlight the gruesome reality of war and reject the suggestion that war is noble or heroic.
Focusing on the horror of war rather than glorifying the conflict accomplishes several things that make for memorable literature. For a start, the average experience of war is undoubtedly one of horror, not heroism. Most soldiers do not receive medals for bravery, but they do witness the carnage of war. By writing stories that realistically portray the horrors of war, a story becomes relatable to veterans and often serves as a tool to help them process their own stories. For example, The Things They Carried received accolades from former Vietnam soldiers who felt that O'Brien captured in his anthology feelings that they struggled to communicate themselves.
In contrast, literature that emphasizes the heroism of war can often come across as propagandistic and does not generally age well. Many action-stories of World War I heroes were published in the 1920s, but few are remembered today. This is because a hero is rooted in a specific historical and cultural moment, whereas a story that explores the terror and horror of conflict can appeal to a broader audience across time and geographic location. For example, Stephen Crane's novel The Red Badge of Courage is just as relevant today as when it was written after the Civil War.
In the case of Tim O'Brien's work, he explicitly attempts to tell a "true war story." In the chapter "How to Tell a True War Story," O'Brien claims:

A true war story is never moral. It does not instruct, nor encourage virtue, nor suggest models of proper human behaviour, nor restrain men from doing the things they have always done. If a story seems moral, do not believe it. If at the end of a war story you feel uplifted, or if you feel that some small bit of rectitude has been salvaged from the larger waste, then you have been made the victim of a very old and terrible lie.

This thought from O'Brien is perhaps the best argument for the effectiveness of literature focusing on the horrors of war: they reflect the truth of violence and the loss that conflict inevitably brings.
I hope this helps!