The First Dominion-American War

WARNING spoilers through Blood in the Water. The First Dominion-American War occurred when three transport ships with US army troopers aboard were transferred from Mexican-American War in 1847. It was mentioned by Captain Anson in Storm Surge [SS Hb. p. 301-302]. More discussion of it is seen in Blood in the Water [BITW chapter 13]. The Americans with Native allies fought and captured the Capital in the Valley of Mexico, but the the overwhelming numbers of the Holy Dominion forces forced their retreat north east into Texas, with the Native allies and wives to form the New United States. After the war the Holy Dominion "cleansed" the valley of Mexico killing all inhabitants who had not fled with the retreating Americans.

Cause
The exact cause of the First Dominion-American War is unknown, as well as who initiated the hostile actions. Considering that the 1847 Americans came directly from the Mexican War of 1847, it is actually possible that they initiated the conflict, by mistakenly thinking that they are still in their own world, and attacking "the Mexicans" (actually the Dominion).

1847 Americans
Unknown. Probably no more than a few thousand soldiers plus unspecified local supporters with superior muskets and artillery.

Holy Dominion
Holy Dominion army and Navy, hundreds of thousands troops.

Conflict
The initial 1847 Americans offensive was highly successful, despite their limited numbers. The main reasons for their success were: By defeating several Dominion armies, the 1847 Americans were able to invade the Valley of Mexico and even capture the old Dominion capital city.
 * The superior weaponry and tactics of the 1847 American troops over the generally XVII-century Dominion army.
 * The element of surprise; the bulk of Dominion armed forces were located on the Pacific, presumably preparing for a war with the Empire of New Britain Isles.
 * The support of local population, who considered the 1847 Americans as their best chance to get rid of the Dom's oppression

Eventually, however, the remaining elements of the Dominion government were able to recognize the threat, and they mobilized all Dominion military and industrial resources against the invaders. The 1847 Americans were able to hold for some time, repelling Dom's counterattacks, but eventually the massive numerical superiority of Dominion troops prevailed, and the 1847 Americans were forced to retreat to the North.

Aftermatch
For some unspecified reason, the Dominion did not attempt to pursue the retreating 1847 Americans. It is possible that the cost of such an operation was prohibitive for the war-torn economy of the Dominion. Instead, the Dominion opted for creating a massive "buffer zone" between themselves and the 1847 Americans, by transferring the capital to the south and basically depopulating the whole Mexican Valley.

The 1847 Americans and some of the local population retreated to North America, and eventually founded the New United States on the northern coast of the Gulf of Mexico. According to Captain Anson the war continued to this day, but the discussion in Chapter 13 shows that the actual situation was close to a prolonged cold war with occasional periods of open hostility - like the Second Dominion-American War, during which Cuba was taken by NUS from the Dominion.