Complex sentence: Nivelle had been concerned about diminished French morale at Verdun; after his promotion to lead the Second Army in June 1916, manifestations of indiscipline occurred in five front line regiments.
Split into simpler sentences: Nivelle had been concerned about diminished French morale at Verdun. After his promotion to lead the Second Army in June 1916, manifestations of indiscipline occurred in five frontline regiments.