After the u. s. made the Louisiana purchase in 1803, the idea of manifest destiny gained strength. Most Americans believed that the u. s. was meant to extend westward across North America from “sea to shining sea.”this belief motivated Americans to settle the wild frontier and claim the land for the United States. Develop a logical argument that supports this opinion of most Americans. After 1803. Use text-based evidence to support your response. Cite at least one source in your answer.