Complex sentence: The Italian contribution amounted to over 60,000 troops at the height of the war, and the involvement helped to increase Mussolini's popularity among Italian Catholics, as the latter had remained highly critical of their ex-Socialist fascist "Duce".
Split into simpler sentences: The Italian contribution amounted to over 60,000 troops at the height of the war. The involvement helped to increase Mussolini's popularity among Italian Catholics, who had remained critical of their ex-Socialist fascist "Duce".