Complex sentence: By then, certain modern historians believe, some 43 members of the garrison were dead or missing for other reasons, but because so many non-combatants were present in the Fort when it fell, the number that died cannot be stated with any precision.
Split into simpler sentences: By then, certain modern historians believe, some 43 members of the garrison were dead or missing for other reasons. Because so many non-combatants were present in the Fort when it fell, the number who died cannot be stated with any precision.