The main impact of Manifest Destiny was that it helped encourage the United States to aggressively spread across the continent.
Manifest Destiny was the idea that America was destined to spread itself over a wider and wider territory. People who believed in Manifest Destiny believed that the US was the best country in the world. They believed it was superior to all others in terms of its government, its religion, and its society. In addition, it was greatly superior to Mexico and to the Indians in racial terms. Because of this, they felt, the US deserved to spread across the continent.
Not surprisingly, this attitude helped to encourage Americans to take more territory in North America. It helped to push them into the war with Mexico in the 1840s. It helped to justify the taking of the Native Americans’ territories in the time of westward expansion. This, of course, had tremendous impacts on the United States as it helped to make the country much bigger and more powerful.