In the border zone between Mali, Burkina Faso and Niger, French and Burkinabe soldiers march in step. Named Bourgou IV, their mission was the first official joint ground operation between the French army and the so-called G5 Sahel force, which pools troops from Burkina Faso, Chad, Mali, Mauritania and Niger. Their goal: assert control over an area where no troops had set foot for over a year. It is in this "three borders" area that 13 French soldiers lost their lives in a helicopter accident on Monday.