Complex sentence: A skilled front-line officer, Kopański did not have much work "behind the desk", especially since, while most of the Polish units were fighting under Polish command, they were part of Allied fronts and armies.
Split into simpler sentences: A skilled front-line officer, Kopański did not have much work "behind the desk". Especially that most of the Polish units were fighting under Polish command, but as part of Allied fronts and armies.