Manifest destiny

Manifest Destiny was the concept that the United States had a God-given right to claim territory across North America. The phrase "Manifest Destiny" was created in 1845 by a newspaper writer named John L. O'Sullivan. Quickly, the idea became very popular around America.
Definition.
The idea of Manifest Destiny told Americans that they had a mission, a special job given only to Americans. This mission was to make the United States bigger so the "borders of freedom" could be open to even more people. Only Americans could make sure that people living in not-free countries across the world could learn how to govern themselves and live freely in America.
Use by President James Polk.
In 1845, James K. Polk was elected President. Polk believed very strongly in the idea of Manifest Destiny. During his Presidency, the United States grew by a million square miles.
The Mexican-American War.
Ever since he was running for President, Polk had talked about wanting the area that is now around Texas. At that time, Mexico controlled that land.
In June 1845, the United States decided to add Texas to the United States. President Polk sent the United States military to take over the area. The two sides fought for two years. Finally, Mexico surrendered. On February 2, 1848, it signed the Treaty of Guadalupe Hidalgo, which ended the war. It also gave the United States all of the land above the territory. This meant the United States had gained all of Texas, Arizona, and New Mexico; parts of Utah, Nevada, and Colorado; and northern California.
A belief in Manifest Destiny helped convince Americans to support the war. It told them it was all right to fight, kill, and take land from Mexicans or other non-Americans. This was all right because they were only doing what God wanted. They were doing what they were destined to do.
Settlement of the west.
The belief in Manifest Destiny was also important in encouraging people to go to other parts of the west, like Oregon and southern California.
Problems.
The United States was very determined to take control of all of North America. Unfortunately, the parts of North America they wanted were not empty. They were places that Native Americans had always lived.
The United States government wanted these tribes to move outside the United States, to what is now Oklahoma. The government was able to convince some of these Native American tribes to sign treaties saying they would leave their lands.
When some tribes refused to leave their lands, the United States government committed an ethnic cleansing against them. In an ethnic cleansing, ethnic or religious groups are forced to leave an area by a more powerful ethnic group. When the Cherokee Nation refused to leave their homes, United States President Martin van Buren ordered the military to make them leave.
About 15,000 Cherokee were forced to walk to Oklahoma and Arkansas. Most historians think that about 4,000 of them died on the way. This was one out of every four people in the Cherokee population. Because so many people died, this forced migration is now called the Trail of Tears, and it is considered an ethnic cleansing.