Complex sentence: The Division was under command of the XVIII Mountain Corps and under attack by the advancing Russian forces in the Karella sector, they were ordered to breakthrough at Kuusamo, Kistinki and simultaneously to prevent reinforcements moving up.
Split into simpler sentences: The Division was under command of the XVIII Mountain Corps and under attack by the advancing Russian forces in the Karella sector. They were ordered to breakthrough at Kuusamo, Kistinki and simultaneously to prevent reinforcements moving up.