The ACMF had been an extremely large threat and force pre-war. Having binded most of Asia together, Russia and China were at the helm of it. The ACMF had launched a large-scale coastal invasion on the USA before the nuclear war had struck, but the nuclear strike destroyed the ACMF, leaving most of the soldiers who made it through on the coastal invasion in the wastelands of the USA. They are enemies to everybody simply because they were the reason for the nuclear strike and they're stranded in their enemy's homeland. They own the Pre-War Town Ruins; which was bombed heavily during the raid. They are stranded military operatives from the raid on Florida during the final days of the war in 2032. The USCPF and the ACMF do not take kindly to each other.
Countries such as North Korea were less involved with the US fight, as they were busy with NATO wanting to take control of the border between North Korea and China. China had previously pushed NATO back with several successful attacks. The US had promised North Korea that missiles would not be fired close enough to cause radioactive damage.
After the war would've occurred, large amounts of stranded ACMF operatives would band together and take over small camps, still struggling to be the force they once were. These remnants tussled often with the USCPF, and in most cases, lost more members.
Post war, many descendants would've came from deserted ACMF. A large percentage of them would never come to know their history, aside from a few. Being an ACMF descendant was a shrouded piece of their history, as ACMF was still blamed along side the United States for causing the flash in the first place. Many descendants would not hear of their origin until they were adults.
In 2045, the submarine was abandoned by the people maintaining it they had left it in a cove which, for a time, was relatively undisturbed. The submarine was underwater for a large portion of its existence. A few scavengers here and there found it, but up until Whitman Bay was reformed, most were killed by the remaining ACMF or just couldn't get in. Eventually, those ACMF left the submarine, and had a few kids. Those kids took up finding the submarine, and that's where it is now. -Credit to jasonthekiller for the submarine lore.
Please note: A few fisherman did find it, but for the most part, stayed away from popular rumours or couldn't get in. Plus, a lot of the items in the submarine are either stolen or bought. They aren't all pre-war items. -Credit to Skelethon.