Manifest Destiny had a big effect on expansionism both at home and abroad. Manifest Destiny refers to our desire to expand from the Atlantic Ocean to the Pacific Ocean. Once that was accomplished, it took on an international form of expansion. There are several examples of Manifest Destiny at work.
There are several examples of domestic expansion. When Texas got its independence from Mexico, we wanted to annex Texas. While annexation was delayed for nine years because of slavery concerns, there was little doubt Texas would eventually join the United States. We also wanted to add the Oregon Country to our land. While there was some desire to add all of the Oregon Territory, going from what is now the northern border of California to the southern border of Alaska, we reached a compromise with Great Britain that split the territory at the 49th parallel. Thus, we added what is now called the Pacific Northwest to our country. Finally, we had our eyes on what is now the southwest region of the United States. We went to war with Mexico to add this land to our country.
By 1853, we had acquired all the land that currently makes up the lower 48 states. People began to believe we should also expand internationally. Since almost all desirable lands were already colonized, we would most likely have to go to war in order to expand beyond our borders. We ended up fighting Spain in the Spanish-American War in 1898 to accomplish this goal. As a result of this war, we got Puerto Rico, Guam, and the Philippines as United States colonies. We now had taken the idea of Manifest Destiny to an international level.