Notice how carefully I worded my comment:
>70% of the land is not the U.S. Government's to give.
"The federal Indian trust responsibility is [...] a legally enforceable fiduciary obligation on the part of the United States to protect tribal treaty rights, lands, assets, and resources." 
The questions of armies and building bases I'm not sure have been raised, so the legal precedents might not exist. Sufficed to say, however, that the GP's hypothetical sale to Canada would be wholly infeasible in light of the treaties between the sovereign tribes and the US, as well as the legal complexities those entail.
"Limitations on inherent tribal powers of self-government are few, but do include the same limitations applicable to states, e.g., neither tribes nor states have the power to make war, engage in foreign relations, or print and issue currency."
I'm not sure if this would prohibit raising an army per se, but what good is raising an army if you don't have the power to make war? Building bases is wholly out of the question, though, since that would involve engaging in foreign relations.