"...and establishing his headquarters and reserve arsenal at Indio, where the California National Guard had been consolidated at makeshift barracks along the rail route to the Colorado. Pershing appointed Charles Menoher, a former West Point classmate, as his chief subordinate, and was asked by California Governor Hiram Johnson to wait to press on offensively until the commander of the National Guard was dispatched. Upon hearing that said commander would be a political appointment - Congressman Joseph Knowland, who represented a district centered upon Alameda County on the San Francisco Bay - Pershing elected to press ahead. As American defenses in Maryland seemed to evaporate at the slightest contact from Confederate forces, a victory needed to be secured on some front, somewhere.
Yuma was the natural target, both for Pershing's gathering battalions in Indio and from a broader strategic standpoint. Word that Mexico had declared war on the United States arrived at Indio and it did not take too many glances at a map of North America's strategic rail network to figure out where they could make an impact. From railheads at Nogales and Los Pasos, Mexican forces could fortify the Confederate West and, potentially, harass or even attack New Mexico or, less likely, California. One priority for Mexico would almost certainly getting some of their troops to the isolated northern outposts of Baja California and the carnival town of Tijuana on the border [1], which Marines from San Diego had rapidly moved to seize within days of the declaration of war with minimal resistance. The immediate, if minor, threat to America's position in the West thus came from access to the Colorado and its delta, control that flowed through Yuma.
The Confederacy had done considerable work over the past decade in making Yuma into a thriving commercial town; though the frontier of both countries was definitively closed, the West was an area of growth and opportunity. Until the trade wars of the early 1910s, Yuma had been the main conduit for Confederate wares and goods to the Pacific, either down steamboats on the Colorado into the Gulf of California or, eventually, by rail to Los Angeles or San Diego. It sat at the confluence of the Gila with the larger Colorado and the tripoint border of the Confederacy, United States and Mexico. The logistical value of the city was thus considerable, and it would be a target that would certainly grab Richmond's attention. On September 20th, at the same time that the Army was forming its official theater commands from the wartime capital in Philadelphia [2], Pershing had his railcars collected and gave the command to move forward to attack the following day.
Yuma was defended by a namesake fort on the banks of the Colorado itself, where the railroad crossed the river, garrisoned by approximately two thousand men. Early on the morning of the 21st, Pershing's mobile artillery opened fire across the Colorado, the first pitched battle of the Southwestern Front. One of the advantages the Confederacy did have, compared to their attackers, was a robust fixed artillery position and machine gun nests at Fort Yuma that allowed them to screen the rail crossing at the river. Pershing pressed ahead anyways, hoping that his suppressing fire would be sufficient; Menoher expressed skepticism at this maneuver but gave the orders anyways.
The Battle of Yuma was thus typical of a Pershing battle; aggressive and with willingness to take casualties in order to overwhelm the enemy with relentless force. [3] Two thousand American casualties were sustained in the crossing of the Colorado, with the typical 25% killed-in-action ratio. However, Yuma fell before nightfall as the artillery support eventually destroyed two of the de Bange 90mm guns, diminishing Confederate defensive capabilities, and most of surviving Confederate delegation surrendered rather than fight on once the bridgehead was established. Pershing ordered the California Guardsmen transport the prisoners back to Indio, which would be the site of "Camp Pershing," a major Confederate prisoner-of-war camp later converted to a fort in the aftermath of the war.
The first major victory for the US of the war had been won - the Colorado was now entirely under the control of the United States, and the threat to California or western New Mexico effectively negated within the opening weeks of the conflict..."
- Pershing
[1] Even in the early 1910s, Tijuana basically existed for Americans to come to engage in various forms of debauchery
[2] "Wartime"
[3] This is something Black Jack was kind of infamous for in WW1; it wasn't quite the human waves of the Soviets in WW2 but he was a huge fan of frontal attacks. Pershing was a mediocre tactician but a very good political general in terms of his role as head of the AEF, IMO, but the conditions in the Southwest are very different from the Western Front of 1918 so his more aggressive offensive style would work better here.