What is the object of the prepositional phrase that appears in capital letters. About 720000 Americans died DURING THE CIVIL WAR?

The object of the prepositional phrase "DURING THE CIVIL WAR" is "WAR." In this phrase, "DURING" is the preposition, and "THE CIVIL WAR" acts as the noun phrase that serves as the object of the preposition, specifying the time period in which the events occurred.