Drill Shoujo Yui

In the beginning of the 21st century large-scale terrorist activities are carried out by the remnants of the research team survived after the World War II.
Thanks to international special unit the activity of the group is put to an end. Protagonist is the retired member of this unit who travels around Japan 10 years after the incident. He is approached by a woman who proposes to create a new special unit to trace the new suspicious organizations. Protagonist refuses, but changes his mind after seeing member candidate - a young woman who lost relatives in the terrorist attack ten years ago. His life as instructor begins.

[From vndbreview]