In studying the history of New Mexico's involvement in the American Civil War, it is unclear to me if, as just a territory at the time, and not a state, it could officially "support" a side? Nearly all maps showing the split of the states, don't indicate which side the territories supported. It seems that unofficially, those living in the southern half of the state tended to support the Confederacy, due to economic ties in the cattle industry with Texas. Yet those in the northern have tended to support the Union. But these trends don't represent the government of the territory picking a side, just attitudes of the people. Then there was the regions taken in actual military conflict, but that still is different from the territory joining a side. I have some history books on this, but I am suspicious the authors are not fully forthright, because admitting a territory joined the Confederacy is controversial. Did the territories pick a side in the war?
Legally, a territory is directly governed by the federal government, and can't pick a side. There's no possibility of "withdrawing from the Union" or equivalent.
In practice, things were a bit different:
- Colorado Territory: Mixed sympathies, sparse population, and no overt declarations.
- Confederate Arizona: The southern portions of Arizona and New Mexico Territories supported the Confederacy and established a pro-Confederacy government in Tucson.
- Dakota Territory: Very sparsely populated.
- Idaho Territory: The eastern portion sympathized with the Confederacy; the western portion was pro-Union. This was the impetus for dividing off the eastern section into Montana Territory.
- Indian Territories: The tribes of the Indian Territories allied themselves with the Confederacy.
- Montana Territory: Confederate sympathies, but no declaration of support.
- Nebraska Territory: Strongly pro-Union.
- Utah Territory: Anti-Union, but not pro-Confederate.
- Washington Territory: Generally pro-Union.
Overall, approximately two territories joined the Confederacy, while Nebraska contributed a large contingent to the Union army. The other territories had general sympathies towards one side or the other, but didn't explicitly pick a side, becoming Union by default.