B. Generals were slow to change military tactics in response to new weapons with longer fire range.
One of the main reasons that the Civil War was more deadly than any previous war was because of the use of new weapons and technology, such as the rifled musket and repeating rifle, which had longer fire range and greater accuracy than previous weapons. However, the military tactics used by generals had not yet adapted to these new weapons, and soldiers were often ordered to attack in mass formations, resulting in devastating casualties. This is why option B, that generals were slow to change military tactics in response to new weapons with longer fire range, is the correct answer.
Option A is not correct because an all-out charge of troops was not a new tactic and had been used in previous wars.
Option C is not a direct reason for the higher death toll, as soldiers fighting for a cause they believed in was not unique to the Civil War.
Option D is also not correct as ironclad ships were not a major factor in land battles and the high death toll on the battlefield.