Complex sentence: It was the responsibility of the Brazilians to win the most combative sectors of the entire Apennine front, but the 1st DIE was too inexperienced a troop to face a fight of that magnitude.
Split into simpler sentences: It was the responsibility of the Brazilians to win the most combative sectors of the entire Apennine front. But the 1st DIE was too inexperienced a troop to face a fight of that magnitude.