Little is known about the shadowy paramilitary group which is believed to be linked to Russian oligarch Yevgeny Prigozhin, who is himself an associate of President Vladimir Putin.
The presence of Wagner paramilitaries has been documented in Libya, Mali and Syria, among many other countries, as well as in Ukraine.
Naming the group Mozart was “a tongue in cheek” retort to the group, according to Andy Bain, a former United States marine reserve officer.
Mozart – which was set up after the war started by an ex-US commander and is funded by donations – provides military training, but Bain says what they teach is really “a lot of common sense”.
“A lot of these soldiers have never fired a weapon before,” says group chief operations officer Martin Wetterauer.
“Weapons are obviously very dangerous in the hands of people who don’t know how to use them.”
IT’S A GUN, “NOT A GUITAR”
In a field in the Donetsk region, Mozart’s foreign instructors take a group of around 20 Ukrainian soldiers through their paces.
“The enemy is there! Bang, bang, bang!” an instructor shouts from the other side of the field as the soldiers advance into the open.
The soldiers swiftly throw themselves on the ground and simulate firing back.
After the exercise, their performance is reviewed by instructors who try to correct mistakes.
“What’s so complicated? In essence, what you have to do is just fire at the enemy!” one of the coaches shouts in English.
“Don’t hold your weapon like this, it’s not a guitar!” his interpreter tells a soldier.
Georgii, a 32-year-old soldier in training, says he feels that his skills have improved through the course.
“That kind of training is very useful because even with combat experience, we can always learn new stuff,” he says.
But a lot of the soldiers are beginners.
Former Marines officer Wetterauer explains that a lot of Mozart’s training revolves around “battlefield survival ability” and “basics training”.
This includes knowing how to wear a bulletproof vest correctly, protecting yourself from enemy artillery by digging shelters, or providing medical care.
“We have a very small impact strategically on the outcome of this war, we know that,” Wetterauer says.
“But for us, it’s about saving lives.”