While tensions between Korsgaardista Mexico and the Dominion of Southern America had been rising just prior to the Mexican invasion, and several expatriote Mexican groups had made claims that the Mexican Empire would attack, the sheer audacity and scale of the MExican invasion still took the Dominion of Southern America by surprise. The troops sent to man the very long border between the Dominion and Mexico had never been adequate to assure true security, and thus despite the many acts of bravery in the early days of the Dominion's invasion, the Mexican forces made heavy inroads along their three spearheads.
The second line of defense in those early days were hit and run raids in a running retreat made by the Royal Southern American Rangers, and in New Mexico, the unlikely allies of the wild tribes there who took payments from the Provincial governments to raid the Mexican supply lines.
New Mexico had always been sparse in population, and thus the Mexican army was able to cut deep into the dry land along the old Mexican road to Santa Fe, in the process cutting off the railways to Albion. Albion itself, especially the city of San Diego, were targeted and occupied to the south, though attempts to penetrate the Central Valley were met by hostile resistance in the mountains by the farmers of that region.
Texas was the most populated of the invaded provinces, and the occupation of the southern regions of Texas wore hard on the proud Texans. The Mexicans were stopped at the Battle of San Antonio (often misidentified as occuring at the
City of San Antonio, when in fact it occurred along a line stretching from that city along the
San Antonio River to
San Antonio Bay. Along this line an impromptu army of British Army regulars, Dominion auxilliaries, Rangers, and volunteers held the tide against the Mexican forces. The Line of San Antonio would go down as a glorious moment in Texas history.
With the Global War engulfing the world, the British relied on locally raised and armed Southerners to take the fight to the Mexican Empire. Local recruitment provided more seasoned troops than might be obvious at first glance, as many Southerners spent some time in the British Armed Forces before returning home to the Dominion. The locally raised Armies of the Provinces would prove decisive in North America, especially in the counterattack to free occupied Texas.