Sino American war crimes and timeline
Oil starts to run out.
The US leaves NATO and becomes isolationist.
The Soviet Union, Western European alliance, and Middle East destroy each other.
A now defenseless Taiwan, Japan, and South Korea is easily conquered by China. China then invades Alaska with a massive military force as Alaska was the last place on earth with the largest known oil reserve. This gives the US justification to annex Canada. Canadians who protest American rule are killed on the spot.
In an attempt to find an alternative to oil, America had already invented nuclear fusion and implemented the tech into their war machines including armored vehicles and a super heavy body armor known as power armor allowing them to take back Alaska.
Nuclear fusion would have saved the world from it's energy crisis but the technology was still new and expensive meaning it could not be scaled immediately for global use.
China responds to their defeat in Alaska by launching bio weapons against American military units and civilian populations.
America responds by invading mainland China itself after defeating the Chinese navy in a brutal war over the Pacific. The Chinese commit scorched earth warfare in Taiwan, Japan, and Korea to prevent America from using them.
America starts its counter bio weapons research program in California, the fearsome Forced Evolutionary Virus. The goal was to turn soldiers into "super humans" that were immune to pathogens and more physically capable than regular humans.
Mice, dogs, and chimpanzees weren't enough, the US needed actual human test subjects. US government starts human experimentation on Chinese prisoners of war, Canadian dissidents, and criminals. Thousands of human test subjects were now subjected to brutal experiments and death from the initial tests. The US government also poisoned water in small Appalachian towns with the virus to see it's effects. When Americans in those small towns were turning into mutants due to the virus, the US quarantined the towns, arrested everyone, and threw them into underground labs for research.
In an attempt to turn the tide of war, China had already invented invisibility cloaking and sends in invisible spies to sabotage America's war effort/industry and they are able to use bio weapons on American military installations and civilian populations more effectively. One of the Chinese bioweapon is a modified smallpox virus made by the Soviets that was killing millions of Americans as there was no vaccine against this artificial strain of smallpox.
Hearing what was occuring back home, US troops in China begin gunning down Chinese civilians as revenge. US government also starts shipping captured Chinese civilian populations to be used as test subjects. Hell and chaos erupts, society and order starts to collapse as the war intensify, death toll rises, and atrocities are committed by all fronts
China's fleet of stealth/invisible submarines are still relatively untouched. Fearing defeat might be near, they are ordered to sail for America.
The US military with nuclear fusion equipped weapons allows them to defeat any conventional Chinese forces thrown at them as they close in on Beijing. A single American soldier inside of power armor could take down hundreds of Chinese troops or several Chinese tanks by himself before going down.
The Forced Evolutionary Virus research is now going well after tens of thousands of subjects had died. Test subjects became mutants but they were immune to the diseases and torture that's inflicted on them by the US scientists and saw rapid cell regeneration.
US troops guarding the perimeters of the research lab had been kept dark about human experimentation but when they discovered what was going on, the captain in charge who was one of the few left on earth that still had any morals deemed it a human rights violation. The scientists who were sanctioned by the US government to test on humans were rounded up and executed. The captain and his unit declares secession and waits for a final battle/death against the US government but..
Refusing to surrender or be defeated as Beijing was about to fall, the CCP orders their stealth submarine fleet who had by now surrounded America's west and east coast to launch their nukes on all major US cities/towns.
America launches nukes back at China as the CCP and US government both go into hiding. Billions die from the war and it's aftermath from famine and lack of medicine. Knowing that the world had ended, the captain and his unit at the research facility salvaged what they could, took in any civilians that would follow them, abandoned the base, and established the Brotherhood of Steel.