Complex sentence: There were far more Soviet anti-tank guns, minefields and anti-tank ditches than had been anticipated, making a breakthrough more difficult to achieve, and they faced frequent counterattacks from tank units brought up from the Front's reserve.
Split into simpler sentences: There were far more Soviet anti-tank guns, minefields and anti-tank ditches than had been anticipated, making a breakthrough more difficult to achieve. Further, they were subject to frequent counterattacks from Soviet tank units brought up from the Front's reserve.