And the next part...
The Mexican and Southern Alliance's forces battled eachother to a standstill, as the Mexican-Califurnyan Armies' combined strength was actually enough to match their rivals' power.
That was, until
they came.
In America's absence, a new power,
the Empire of the Rising Sun, had emerged in the Japanese Archipelago. With the USA's de facto dissolution, the Japanese nationalist-dominated Diet dismantled all of the treaties that the USA had signed with them, from mutual defense to the renouncement of war as a means of foreign policy. Japan's foreign policy goals could be summed up in one word, according to the greatly-empowered Japanese Prime Minister: "Freedom." Freedom to do as the country chose, anyway.
And accordingly, it was the Empire of the Rising Sun that seized opportunity. It transformed Japan's advanced civilian sector into one capable of efficiently and cost-effectively waging war, with little dependence on raw materials. Japan's war machine became quite frightening, as it relied on using what Japan already had rather than acquiring more through programs such as renewable energy and recycling. In the two decades since the USA's dissolution, the Empire managed to conquer much of South Korea(many Koreans chose to rush into the arms of the Japanese than the North Koreans and Chinese, who invaded at the same time as the Empire). Adding to this were victories in Southeast Asia, Indonesia, and the Pacific. Within two decades, the Empire had asserted formal or informal control over nearly every area in the Pacific region sans Australia, China and Russia.
Shortly after the Hawaiian government was overthrown by a group of revolutionaries, these revolutionaries appealed for annexation by the Empire. This was a sign of things to come. And immediately after, the extremely powerful Imperial Navy sailed eastwards at top speed, and soon citizens all the way from North Cascadia to Southern Mexico found their homes and cities assaulted by the Eastern forces. The Empire declared a unilateral alliance with the Southerners, and before long it had occupied nearly all the Hispanic Alliance's islands, as well as cities such as San Diego. The Cascadians were surprised to lose Vancouver as they were neutral, but being friendly - and having a defensive alliance - with Califurnya, it was only natural the Japanese would attack them too.
The sudden attack by the Japanese prompted the Southerners to renew their advance, and the result was a massive loss of Mexican territory in the north, while Louisiana successfully launched an invasion across the Gulf of Mexico into Cancun, and the Southeast Confederacy managed to seize Havana.
While all this went on, the rest of the continent remained neutral, even when the Japanese ambitiously attacked Alaska for no apparent reason, supposedly over a boundary dispute over the Aleutians and other Pacific islands. The USA itself expanded further by buying the island of Bermuda from Britain, which was in a deep financial crisis thanks to numerous European issues.
But, could the Imperial forces be trusted? Sure the Japanese seemed to merely only desire the Pacific, but, like all expansionist monsters, what was the stop them from wanting... more?