When the American Indians were forced to move to Oklahoma.
Explanation:
Andrew Jackson hated Indians, so during his presidency, he made Indians move out of their homelands. The Trail of Tears was when all the Indians had to walk from their homeland to Indian Territory, or Oklahoma.
Natural elements of the ecosystem are often borderlands, for example rivers or mountains, the Great Lakes.
Easy escape from potential threats
The docking of America and Solviet Spacecraft.