Washington state weathered the Collapse better than any other place in the country, which is what killed it.
After the rotting plague came famine, strife, and other, lesser plagues. Starvation makes one vulnerable to infections, and in the absence of a robust medical system, the spread of disease can be fast and deadly.
Other than Seattle, Washington state managed the Collapse very well. The state government established refugee centers where people could come to be fed, sheltered, and receive a modicum of medical care. These centers enabled many to survive the Black Winter of 2015-2016, when they might otherwise have died.
They also concentrated much of the populace into tight clusters. Secondary diseases, when they came, hit the camps hard. When they arrived, cholera, dysentery, and the seasonal flu spread rapidly, killing millions. In the space of a few months, Washington lost more people to these lesser afflictions than it had to the rotting plague.
In all, the state suffered 90% casualties. By the end of 2016, there were less than 700,000 Washingtonians left alive. When the Communist Party exiles began arriving in late 2018, the state was overrun.
The million refugees from the collapsing Communist nation were slowly evacuated over the course of five years, on a multitude of different vessels (powered with fuel bought at dear prices from the Siberian junta). The top officials were evacuated on luxury liners, while the poorest were stacked into cargo containers in the holds of massive ships (that had once carried plastic toys to Wal*Marts). The soldiers came over on military ships and pretty much anything else that floated.
The city gradually grew in size and power, as more and more citizens, soldiers, and military vehicles arrived. It began to push outward into the depopulated Washingtonian countryside, first to the greater Seattle metropolitan area, then south to Olympia, west to Aberdeen, and north to Bellingham.
Starting in 2019, the state faced increasing military pressure from several surrounding enemies: Outlaw settlements and roaming gangs in Washington, the militia of the hardline state of Jefferson in Oregon and California, and several hundred thousand well-armed, peevish Canadians in Vancouver. (Peevish by American standards, that is, which made them rampaging barbarians by Canadian standards. Vancouver had become their capitol, after all.) Border spats with the two polities became increasingly common in 2020, and the Chinese were hard-pressed to maintain their territory.
That same year, they negotiated a trade treaty with the Fed (which effectively recognized Xiyatu’s sovereignty over most of Washington), which allowed them to buy ammo, replacement parts, and fuel with the bullion and other artifacts brought from China. This trade gave Utah’s arms industry its kickstart (leading to the popularization of the AK-47) and helped Dakota expand its nascent industrial base.
When the Chinese industrial base began to normalize in 2021, Xiyatu became one of its most loyal clients. It imported nuclear reactors, for electricity (and other purposes), and electric automobiles (for use of the elites). Due to this trade, Xiyatu became a major port city, the only continental source for computers (manufactured in China), and the primary export hub for American raw materials and finished goods.
Nominally Communist, in practice the city was an authoritarian polity, with a partially-liberalized economy (similar to New York and California). The ruling clique had a large number of perquisites, and there was severe restrictions on the press and non-State organizations, but other than that the economy was fairly unregulated.
Of course, all this changed after Emergence, when a clutch of dragons settled in the nearby mountains, and successfully claimed Xiyatu for their own. And with the dragons, magic.