In “ambush,” why does the author begin the passage in the present rather than during the war?

The correct answer here is to build tension in the story.

Creating tension can be really tricky and authors use a number of techniques in order create tension in their works and grip the readers from beginning to the end. One of the ways to achieve that is the sequence of events. And here in order to build the tension the author uses the sequence of events. The tension is higher now as we jump back into the war.