The Film follows the events of Leave the World Behind.
The media started the war.
By launching a smear campaign against the President, they spread distrust around the community and now they have blood on their hands. All because they wanted a raise in viewership.They really don't need to conquer the Earth themselves. They just needed to push a few buttons here, pull a few strings there, and humanity will eventually be at each other's throats. The President is Ron Swanson
He worked his way up from Pawnee, Indiana and dismantling the FBI seems like something Ron would do. The US balkanizes after the President dies
Even though the Western Forces killed the president and arguably have military superiority, the remaining states are unwilling to subsume themselves to them or the western forces are simply unable to force them to capitulate and the war drags out for many more years, destroying what's left of the United States. The film depicts an alternate reality where the January 6th insurrectionists were successful.
Other nations, or at least the UK, are sympathetic to the Western Forces.
There was a British journalist embedded with the Western Forces during the Battle of Washington D.C. It is possible that the UK (and probably other countries, given international alliances) were sympathetic to at least the WF. It would also explain why the President didn't flee Washington before the battle, since it is possible he would have risked arrest overseas and extradition to the winning factions.