Complex sentence: They remained at positions till 8 July and took up offensive again with the approaching Soviet units, but having incurred heavy losses under bombardment from air and artillery, they had to withdraw to Szwajcar.
Split into simpler sentences: They remained at positions till 8 July and took up an offensive again, together with the approaching Soviet units. However, having incurred heavy losses under bombardment from air and artillery, they had to withdraw to Szwajcar.