Is there sufficient information in the Fallout universe to infer who launched Nukes first?
From Fallout Wikia:
It is not known who launched the first nuclear weapon that started the conflict. President Richardson would tell the Chosen One that China launched first; however, it is not known if he was telling the truth. Captain Zao was in the vicinity of Boston at the time of the Great War. However, possible Chinese bombers were spotted off the Bering Straits of Alaska at least as early as 3:37am Eastern Time. Enemy nuclear missile launches were confirmed by NORAD at 9:13am, with U.S. bombers scrambled and airborne at 9:17am. Nuclear missile strikes on New York and Pennsylvania were confirmed at 9:42am, with nuclear strikes on Boston and Washington occurring at 9:47am; this information and multiple reports of U.S military forces advancing across the Chinese mainland and occupying Shanghai and Nanjing makes it likely that the Chinese were the first to initiate a nuclear strike.