The spirit of manifest destiny showed itself in the American expansionism of the 1840s because manifest destiny was the idea that the United States was destined by God to expand across the entire continent of North America.
The idea of manifest destiny held that the United States was a superior nation chosen by God to be great. Believers in the idea felt that the United States had the best system of government in the world. They felt that Americans were racially superior to the Native Americans and to the Spanish in Mexico. They felt that American Protestantism was superior to the Roman Catholic faith practiced by the Mexicans as well.
These attitudes led to the American expansionism of the 1840s. They led to the idea that the US deserved to take much of the territory of Mexico in the Mexican-American War. They also led to the idea that the US deserved the have things like the Oregon Territory.
Manifest destiny was an expansionist idea that was used to justify the American expansionism that occurred in the decade of the 1840s.