It was called isolationism :D
Answer:
In 1818, the United States and Britain agreed to occupy Oregon Country jointly. Spain and Russia had few settlers there, so they withdrew their claims to Oregon Country region's vast forests, trapping animals and living off the land. They were known as mountain men
1836<span>Remembering how badly the Texans had been defeated at the Alamo, on April 21, 1836, Houston's army won a quick battle against the Mexican forces at San Jacinto and gained independence for Texas. Soon after, Houston was elected president of the Republic of Texas</span>
The rivers allowed a good system of communication between the tribes and they were a good source of food & transportation.
There is no letter that I see, therefor, I cannot answer this question
