The American Civil War was not only the costliest war the United States ever fought, with more than 200,000 deaths, and also a horrific combination of industrially driven weaponry and primitive pre-modern medical procedures.
The American Civil War was not only the costliest war the United States ever fought, with more than 200,000 deaths, but also a horrific combination of industrially driven weaponry and primitive pre-modern medical procedures.
The American Civil War was not only the costliest war the United States ever fought, with more than 200,000 deaths, and also a horrific combination of industrially driven weaponry and primitive pre-modern medical procedures.
The American Civil War was not only the costliest war the United States ever fought, with more than 200,000 deaths, but also a horrific combination of industrially driven weaponry and primitive pre-modern medical procedures.
The paired structure "not only ... but also" frequently appears on the GMAT. The two pieces must come together: one without the other is incorrect.