The term "Manifest Destiny" was first coined by a newspaper editor, John O'Sullivan, in an article promoting the acceptance of Texas as the 28th state. There was a great deal of discussion about Texas and statehood. There were supporters on both sides, for and against.
Americans steadily moved westward, first through land occupied by Native Americans and then through land owned and occupied by Mexicans and Native Americans. As more Americans moved westward, people developed beliefs, labeled Manifest Destiny in 1845, about how far the United States should extend and why, and ideas of the best ways to use the land.
Manifest Destiny was specifically applied to Mexicans and Native Americans. The doctrine was used to justify the Mexican-American War, resulting in the acquisition of about 40 percent of Mexico's territory, including California. While the Gold Rush brought to California considerable numbers, it brought mostly Euro-Americans to a place predominantly inhabited by Native Americans and Californios (Mexicans living in California).
Texas had won their independence from Mexico and had become an independent country, or so they believed. Mexico disagreed. They considered Texas part of Mexico, even though a Mexican president, Santa Anna, had agreed to Texas independence. There were constant border skirmishes. Seeking help, Texas had asked to join the Union, but they wanted to join as a slave state. Back then, the United States was divided into slave states and free states. Abolitionists were strongly against slavery, and did not want Texas to join the Union. O'Sullivan, using the phrase "Manifest Destiny", wrote that it was our destiny as a people, and as a nation, to expand west. Finally, in 1845, Congress did approve Texas as the 28th state.
At about the same time, there was a border skirmish with Britain over Oregon. The term "Manifest Destiny" was used again. This time it stuck. People began to believe that any people who could win the Revolutionary War and win the War of 1812, both against Britain and both against incredible odds, obviously, by Providence or God, had a mission.
Not everyone who moved west believed in Manifest Destiny. Some were escaping creditors or the law. Some wanted a better life for themselves and their families. But some, convinced that God wanted them to spread the ideals of the American people, and feeling superior to Mexicans, British, and Native Americans, packed up, joined a wagon train, and headed west.