Final answer:
Manifest Destiny was the belief that the US had the right to expand its territory. O'Sullivan's quote compares the US's right to the air and earth that a tree needs to grow.
Step-by-step explanation:
O'Sullivan's quote refers to the concept of Manifest Destiny, which was the belief that the United States had the divine right to expand its territory and spread its democratic institutions across the continent. He compares the nation's right to the air and earth that a tree needs to grow, suggesting that the US has the right to expand and fulfill its destiny. This quote was popular at the time and aligned with the government's goal of westward expansion.