Complex sentence: Italian surrender is more likely if Italy entered the war in 1941 or later, and less so if the Allies declared war on Italy; the Italian Surrender Level (ISL) determines whether Germany or the USA gain (unbuilt) Italian infantry and surface ships.
Split into simpler sentences: Italian surrender is more likely if Italy entered the war in 1941 or later, and less so if the Allies declared war on Italy. The Italian Surrender Level (ISL) determines whether Germany or the USA gain (unbuilt) Italian infantry and surface ships.