a year ago I did a little world-building for a narrative that was never written, about the planet one day becoming (apparently) tidally locked to the sun with no scientific explanation, nations collapsing from the environmental and climate turmoil, etc. The story would have taken place in the West coast of the united states, which was in evening sunlight when the Sun apparently stopped (and still is in evening sunlight, 50 years later in 2065), where the country is now split into the militarist nationalist Cascadia, feudalist California Free State (consisting of California's old central valley, produces food using migrant labor, ruled by silicon valley billionaires), and the flimsily democratic, agrarian Heartland (officially, the remnants of the U.S. still loyal to the old government), along with clusters of drug family oligarchies and corrupt police states in former central america and southwest united states.
Everything is balanced, for now.
Economically, the California Free State (CFA) and Cascadia have a non-aggression pact. The depend on each other; Cascadia is resource rich and manufactures cheap goods for CFA that it cannot produce itself, while CFA produces the majority of the food and lends profusely to Cascadia to help it build and maintain infrastructure (CFA currency is very stable).
Heartland is a third world nation in comparison to the former two. It mostly produces it's own food and uses mostly old tech (or tech smuggled in from Cascadia). As thus, it has little infrastructure, an inflating dollar, and a resource poor, but stable, military provisional government. Heartland is technically an enemy of the CFA as it wants to reunite the United States and re-democratize, but both are threatened by Cascadia's imperialism.
Cascadia would prefer to annex California, as it is limited in power by having to rely on lending. Since the CFA relies on almost only Cascadian military defectors, private security firms, and conscripted slaves for it's military arm, it would fall relatively easily under Cascadia's increasingly well-armed and ideologically loyal volunteer forces. But it can't go in and take over, as Heartland, intent on making what's left the United States whole again and then forcing Cascadia's military leadership to surrender, would unleash it's most devastating weapon: a working nuclear arsenal.
So things are mostly stable between the countries, except Cascadia annexing some tribal lands in Northern California, and sending spies to the Heartland. If Cascadia got too rough on the CFA technocrats, they would join sides with Heartland, as it sees the former U.S. government as not a huge threat despite it's nukes (as Cali wouldn't be so good to re-colonize with Americans after being nuked) and a great ally, because, obviously, nukes.