Complex sentence: Though it is often portrayed as almost a coda to the major battles between York and Lancaster, it was fought between armies of comparable size and casualties on both sides were very much heavier than at Bosworth, because of the concentrated and attritional nature of the struggle.
Split into simpler sentences: Though it is often portrayed as almost a coda to the major battles between York and Lancaster, it was fought between armies of comparable size. Casualties on both sides were very much heavier than at Bosworth, in part because of the concentrated and attritional nature of the struggle.