Some background history explaining why the borders in Shattered America look the way they do.
In the late 1840's there was a movement called The Bear Flag Revolt by a group of settlers who wanted to create an independent California, separate from Mexico. In our timeline their movement to become an independent nation was ended when the movement's leaders announced their bid to become a part of the Union after ceding from Mexico.
However in this alternate timeline, during California's bid to become a US state in the 1840's, a splinter group of Bear Flag radicals launched a coup in California's fledgling government and rescinded California's request, announcing their will become to a nation independent from either Mexico, or the US, setting the events of Shattered America in motion. 15-20 years later the American Civil War breaks out between the North and the South. California played a pivotal during the conflict, and it's absence from the war leveled out the playing ground for the Confederates. Without California's aid to the Union, the Confederates manage to hold their ground better and break from the Union, causing America to split into three sectors between the North, the South, and West. Abraham Lincoln becomes a disgraced president, going down in history as the man who let the Union fell apart.
In the years proceeding the war, a number of colonist hubs across the Midwest also attempted to establish their independence, though they didn't last very long as they were promptly swallowed up by the larger nations that continued to expand. The United States still achieved Manifest Destiny to an extent, having made a frantic scramble west for it's small gateway to the Pacific. Meanwhile California and Mexico fought a war with eachother over Baja California in 1903, which was the beginning of many acts of hostility between the California Republic and Mexico.
Although the United States itself was less than half it's actual size, they still make up for it through their prowess by colonialism, laying claim to territory around the world, such as Alaska, Hawaii, the Philippines, Cuba, Haiti, Puerto Rico, the Dominican Republic, and Panama., while the California Republic and Confederacy took more isolationist routes. The United States still gets into occasional fights with the Empire of Japan over the Philippine Islands.
(I'd also like to throw out this has nothing to do with Calexit... I do not condone that silly movement in any way, and was writing this long before that was a thing.)