##Part 1

The answer to your question actually lies in the fault of your premise: American Indian Tribes did stand a chance militarily against the United States.^1 Once the United States gained its independence from the British, it was not holding a superior position to Tribes simply by being a the descendant of a European colonizing state, if not militarily, then at least financially. During the American Revolutionary War, Tribes played roles for both sides of the conflict that helped sway the odds. The first treaty the United States made was with the Delaware Tribe to secure trade routes through Delaware territory.^2 After the war, the United States needed to deal with the political fallout regarding Tribes as they worked to build up a new nation rife with its own internal struggles. They did this by forming treaties that sought to uphold relationships with Tribes, but that also recognized the inherent sovereignty of Tribes as treaties are only made with other recognized nations.

It wasn't until the 1820s when the United States began making several domestic decisions that began to alter their perception of American Indians on a fundamental level that departed from the understanding birthed by the Articles of Confederation and early administrations (though the sentiments behind eventual removal and genocidal policies was certainly bubbling before this point). In the 1830s, the federal government acted to remove Tribes from the southeast to west of the Mississippi to clear more land for settling. However, as I just described in this answer, westward expansion was initially a gradual process. This is due to several factors:

1. Indian Tribes beyond the secured territories of the states were still considerably strong compared to the fledgling United States;
2. Indian Tribes within the claimed territory of the United States threatened to jeopardize stability in the union;
3. The new nation had something to prove to the world audience;
4. The emerging legal framework around Indians created loopholes in which conquest/extermination eventually became an ill-suited political and economic issue for the federal government.

Each of these aspects differs in their effects on the decision of the United States to make treaties with Tribes as opposed to all out conflict. Let's consider each.

##War with Indians

Much of the course for the future of the U.S. federal Indian policy was set by George Washington's presidential administration. His administration inherited the political framework bequeathed by the confederation era before the creation of the current Constitution, characterized by inconsistent policy projections for dealing with Indians, and precedent established by British governing doctrine.

War with Indians would prove detrimental on a number of fronts. First, it was expensive. Calloway elaborates on the cost of war discussed by key figures in the executive branch, indicating that negotiations with Tribes in 1789 was already proving to be difficult.

The Indians so resented those treaties that rather than submit to them they would prefer continual war, and in the present political crisis and with an exhausted treasury, Knox [Washington's Secretary of War] warned, that would be "an event pregnant with unlimited evil ... for a protracted Indian war, would be destruction to the republic, under its present circumstances."^3

Second, Tribes were already demonstrating their ability to resist further encroachment by settlers. With a limited national military to defend the United States, states and territories often relied on local frontiersmen and militias to do the dirty work of expansion. The federal government was aware of this and despite their very subtle protests in the name of maintaining federal control over interactions with Indians, they did little to stop renegade settlers since it ultimately supported the agenda of Washington's administration. These infractions, however, caused tensions to flare up between Tribes and the United States. Foreign powers also saw this situation develop and often continued to lend assistance to Indians retaliating against incursions, as was the case in the build up to Little Turtle's War (1786-1795) that occurred as a result of botched negotiations and settle influx into the Old Northwest region.

Because these types of incidents were longstanding, the federal government assumed all authority over interactions with Indians, predicated on the Commerce Clause of the new Constitution. Rather than wage costly wars that the federal government could not support, they forged ahead with diplomacy to attempt to right past wrong and create stronger relationships.

##Philosophy, Law, and Reputation

Even though Euro-American settlers held very racist notions toward Indians, it wasn't a simple matter of displacing Natives wholesale and eradicating us. There was a mixture of philosophical interpretation about the "status" of Indians and where we fit into the Western mindset, political perceptions to maintain, and legal doctrine to satisfy their own societal norms that needed to be upheld when it came to dealing with Indians.

There was an ever present need for a strong national government during the early years of the republic, particularly when it came to the expansion of land holdings and the defense of the frontier. Because the new Constitution vested the authority of interacting with Tribes in the federal government, states saw the benefit of a centralized response to Indian raids and our very presence in the territories they demanded. Commenting further on this, Calloway explains:

Georgia became the first southern state and the forth state in the nation to ratify the Constitution in large part because t was embroiled in a conflict with the Creek Nation. Debates about the sovereignty of the United States acknowledged the sovereignties of Native nations as well as of the states and federal government.^4

However, as we have already identified, the federal government opted to take another route to address the "Indian Problem." Washington and his cabinet desired to see the expansion of the territory under American dominion, but wanted to accomplish this without shedding massive amounts of blood. Besides the cost of war, the Founding Fathers also had ethical concerns over how to treat Indians. These ethical concerns did not stem from a respect for Indigenous ways of living or a recognition of the same level of humanity possessed by "civilized" nations, but did include a humanitarian aspect insofar as Western debates had acknowledged a degree of sovereignty exercised by Native Nations and thus a level of humanity existing among savages who could be "reformed." So how should the federal government proceed?

As the founding fathers sought to secure a place for the new republic among the nations of the world, they also had to determine what was the place of Indians in the new nation. The Indians in a sense were resident "foreign nations." This was their homeland, but they had no desire to become part of the new nation that was being built on it. The United States claimed exclusive sovereignty and territorial control, but how should it deal with the Indian nations? The founders turned to the writings on international law to guide them.^5

It was this turning to international law that resulted in the foundation being laid for the legal cementing Indian policy. Interactions between European nations and Tribes were handled much like any other foreign endeavor between nations. Though Tribes were not considered to be fully independent polities (see: Doctrine of Discovery), the actual process of this involved treaty making to the same degree as occurred between any other nation. Furthermore, treating Indian Nations with a level of international decency would afford the United States a better image on the world stage. "Washington wanted Indian relations in the United States to demonstrate to the world that his nation was the equal of European nations in humanitarianism and waging civilized war."^6 Civilized war ultimately meant "just" war in their eyes and the normal conventions governing war needed to be met, even with Indians.^7

Edit: Fixed some words, added a couple more.