The battle was three days long. It was fought with terrible losses on both sides. Horrified by the bloodshed, the gods forced the Roman and Greek demigods to opposite sides of the country, and manipulated the Mist to be so strong enough so the two camps would never meet and the incident could never repeat, ultimately forgetting about each other. The war mirrored the American Civil War. It is unknown if there is a winner in the war.
Leo Valdez finds Bunker 9 which was used during the war as a base by the Hephaestus or Vulcan demigods, depending on which group used the bunker. It was hidden and forgotten until he found it and revealed it to the rest of Hephaestus' Cabin, along with Jason Grace, Piper McLean, and Chiron. Chiron is revealed to have known about the bunker and tells the demigods a small amount of information about the war and how conflicts in the mortal world can carry over to the world of the gods and demigods. Inside, there are hundreds, if not thousands of schematics, projects, and blue prints (one was a blue print of the Argo II) created and was used as a base of operations during the last demigod war in 1864.
The quest group of the Prophecy of Seven discusses the Civil War, and Jason remarks that most (although not all) Roman demigods fought for the Confederacy. It is implied, although not stated, that most Greek demigods fought for the Union. Hazel Levesque is distraught at the news, being both a Roman demigod and the grand-daughter of a slave.