Complex sentence: The battle eventually stagnated and became a periodic trade of artillery barrages, most probably as a result of casualties sustained in frustrated attacks on heavily entrenched enemy positions that could call in a withering array of supporting firepower.
Split into simpler sentences: The battle eventually stagnated and became a periodic trade of artillery barrages. This was most probably a result of casualties sustained in the frustrated attacks on heavily entrenched enemy positions in control of a withering array of supporting firepower.