That is the question asked by Peter LaFarge in his song "The Ballad of Ira Hayes"...
Gather round me, people, there's a story I would tell,
About a brave young Indian you should remember well;
From the land of the Pima Indians, a proud and noble band,
Who farmed the Phoenix Valley in Arizona land.
Down their ditches for a thousand years the waters grew Ira's people's crops,
Till the white man stole their water rights and their sparklin' water stopped.
Now Ira's folks grew hungry, and their farms grew crops of weeds.
When war came, Ira volunteered and forgot the white man's greed.
The answer is unknown. Despite all that the United States has done to them and their people, Native Americans have been among the most loyal troops to serve under the American flag. And we have done some harsh things to even the Native Americans that served us. The Apache scouts, for instance, that tracked down Geronimo down and persuaded him to surrender were disarmed, arrested and placed on the same train taking Geronimo and his band to Florida.
There were benefits, even during the American Indian wars, for Native Americans to serve with the military. We often forget that Native Americans were not a monolithic nation, but were composed of hundreds of independent bands that often warred against each other. Often Native Americans joined with American forces in a battle against a common enemies. Thus you have Crow scouts riding with George Custer at the Little Big Horn.
Joining the army or being a scout also allowed Native American men a chance to be a warrior. In many tribes, the men's jobs were to raid and hunt. When the tribes were put on reservations by the United States government, the men were no longer allowed to raid and hunt and, in a real sense, were culturally emasculated by the American government. But, as a scout, these Native Americans could still raid with the Army and they could still be men. Further being a scout was a way to get off the reservation and experience some type of life again on the free prairie.
Finally, there were financial benefits as scouts were paid. They were given food, uniforms and weapons. Their families had at least the promise, often unfulfilled, of receiving better treatment.
But, still, it is a mystery of why Native Americans have been so loyal and patriotic to the United States. The American government has done little to earn their loyalty.
Yes I know Johnny Cash had the #3 country hit singing the song, but LaFarge wrote the Ballad of Ira Hayes...