Comanche bro

Again in Chapter 13 we find convenience after convenience. Firstly we discover that the Union has made an advance well beyond what they had in 1862 historically. For instance they have raised "the U.S. mobilized in the Southwest: some 4,900 from Colorado, 6,600 from New Mexico, and 1,100 each from Nevada and Utah, almost 14,000 in total." But it should strike everyone that this number seems a tad inflated.

For instance, only Nevada raised some 1,100 volunteers. But those were raised in 1863, and 1864 respectively. The infantry battalion only ever reached 3 companies! Utah by contrast didn't even muster Volunteers, instead the government trusted the Mormons so little they ordered men from California to occupy the state. Colorado it seems has just had the author take the total number of men mustered historically and put it on (without explaining where they get the weapons that allowed them to be armed in late 1862-63) while doing the same for New Mexico.
The meat of this section though, deals with one particular convenience. In this case it comes from the principal tribe of the Southern Plains, the Comanche. Here we are told that:

When Carson rode to the Walls, the People and their Kiowa allies probably could have fielded 3,000 warriors, which would have nothing on the battlefields of Virginia, Tennessee, and Lower Canada; but 3,000 men with the capabilities of the Comanche and Kiowa were a potent weapon in the correlation of forces between the United States and the rebels, the Americans and the British, and the Mexicans and French, all warring over the breadth of the continent.
There is some evidence the Comanche themselves were aware of the possibility of allying themselves with the U.S. government against the Texans; Comanchero traders from New Mexico apparently had brought word of Sibley’s defeats to the Comancheria, and a small group of about 10 Kiowa and Comanche led by the Yamparika chief Paruasemena (Ten Bears) had talked with officers at Fort Bascom in eastern New Mexico in the autumn. The result, after messages back and forth with Doniphan in Salt Lake was Carson’s mission.
The exact details of the discussion, and what was or was not agreed to, were long disputed: at least some of Carson’s orders were verbal, communicated by Canby on his way out of the Department, and the written orders that were preserved are fairly innocuous: simple instructions to try and get an agreement for the Comanche to give up stolen stock and forswear raiding the Santa Fe Trail, which had reopened after the rebels had been driven from New Mexico. In return, the promise was an annuity, trade goods, and a prohibition of white encroachment into the Panhandle Country, at least from the U.S.-controlled territory to the north and west.

In brief, the United States is able to make a deal with the Comanche at Adobe Walls. This should strike anyone with a passing knowledge of the Comanche as dubious at the very least.

Ever since the Battle of the Antelope Hills in 1858 bad blood existed between the Comanche and the United States. While it had been the Texans who led the expedition, the United States government did nothing to protect the Comanche in the aftermath, and had done nothing to stop settlers from intruding upon their land. Which is what precisely led to the Comanche raiding deeper into Union and Confederate territory, as was their wont.

The idea of the Comanche and the Kiowa throwing in their lot with the Union is also suspect. Not in the least because the Comanche are not much like a nation state (unlike the Kiowa who do have a principle chief who could say he spoke for the tribe) and so an agreement with one band of Comanche, would not necessarily hold with another. This is a perennial problem in dealing with the Native tribes which the US would never understand, much to both sides frustration.

This also begs the question of why the Comanche, to use the authors own words: "why, – would the Comanche need permission or even encouragement to do what they were doing anyway?”

A valid question.

The author sums it up as: For the cost of a few wagon loads of trade goods, it had been a rewarding gambit.

The original question still stands. Why do the Comanche need permission to do what they were already doing, and why would they agree to not even raid Mexico, which the Union would be powerless to stop?

The true answer of course, is simple. The Union must have the complication of policing the Comanche in the south west removed, and a way to curry favor with Mexico. Therefore they must offer an ahistorical olive branch to the Comanche, who uncharacteristically accept it. All in order to further inconvenience the Confederacy by forcing the detachment of forces from Arkansas. While it makes for an interesting plot piece, one can find many reasons to doubt its plausibility.

Convenience rides again.