Dude, we have one of the largest countries in the world in terms of landmass. You don't get that way by sucking at war.
Native Americans were not very demanding enemies to conquer.
They were good at war but few and using inferior weapons. And they were not united, first of all.
By the way the Great Sioux War of the 1876 was a complete failure of a US force twice the size of the Sioux force.
Of course you may explain that you were slightly outnumbered both at Rosebud and at Little Big Horn - but that was only due to lack of any basic coordination between Crook, Terry (Custer) and Gibbon. Their combined forces outnumbered all the Native warriors with a ratio 3000 to 1800.
But Crook came on 16 June - was beaten back, Custer attacked on 26th - and was defeated, Gibbon came on 28th - just to see the dead bodies.
Interesting story is how the number of Sioux and Cheyenne warriors started to grow in American accounts of that defeat.
On the next day morning just after the battle Major Benteen reported that there had been between 1,500 and 1,800 warriors facing US forces of Custer, Reno and his own. But already in the evening he inflated this number to 2,500 in an interview with one of journalists. Several weeks after the battle press was already writing about 3,000 warriors. 3 years after the battle (1879) historians and officers estimated enemy strength at between 8,000 and 9,000 warriors.
The first number given by Major Benteen in the morning after the battle (1,500 - 1,800) is the most reliable one.
Later Native forces grew proportionally to punctured American pride.
We know how many tipis were in the Indian camp and basing on this we can estimate the number of warriors.
The number of tipis with breakdown for each nation and tribe was as follows:
Lakota nation (Teton) was represented by several tribes:
Oglala Sioux - ca. 240 tipis
Hunkpapa Sioux - ca. 235 tipis
Miniconjou Sioux - ca. 150 tipis
Sans Arcs Sioux - ca. 110 tipis
Brule Sioux - ca. 70 tipis
Black Feet Sioux - ca. 35 tipis
Two Kettles Sioux - ca. 20 tipis
Nakota nation (Yankton) was represented only by the group under chief Inkpaduta:
Yanktonnais Sioux - ca. 25 tipis
Other nations / tribes were represented by:
Cheyennes - ca. 120 tipis
Arapahos - several tipis (let's say 10)
Inkpaduta's group included also some members of the Dakota nation (Santee) from tribes of Mdewakanton and Wahpekute. Chief himself was a Santee. But most of his group were Yanktonnais.
Basing on the above given number of tipis, most historians estimate as follows:
1 Sioux tipi = ca. 1.75 warriors, ca. 3 squaw and ca. 4 children
1 Cheyenne tipi = ca. 2 warriors, etc.
1 Arapaho tipi = ca. 2 warriors, etc.
Which means there could be 1810 Indian warriors in the great camp at Little Big Horn. However we should extract from this number casualties at Rosebud (16 June) and those who stayed inside the camp and didn't participate in the battle (especially old warriors were among them).
Compare this to 1316 soldiers & scouts that Crook had, 1200 soldiers & scouts that Custer had, some 500 soldiers & scouts that Gibbon had.
Siouxs and Cheyennes were outnumbered 3000 to 1800 in the campaign of 1876 - yet the US forces suffered major defeats.