 Manifest Destiny was the 19th century belief that the United States was destined to expand across the North American continent, from the Atlantic seaboard to the Pacific Ocean.  First appeared in print in 1839, was used in 1845 by a New York journalist, John L. O'Sullivan, to call for the annexation of Texas.

 John O'Sullivan, a journalist, coined the phrase in a 1845 newspaper editorial about the annexation of Texas, in which he spoke of America's "manifest destiny to overspread the continent allotted by Providence for the free development of our multiplying millions."  It was also one of James K. Polk’s idea when becoming President.

 Was a former Governor of Tennessee before being chosen by the Democratic party to run for President.  He campaigned aggressively for the annexation of both Oregon and Texas.  Polk defeated Clay by a very narrow margin and became the 11 President of the United States

 In 1810s and 1820s, Russia and Spain had given up their claims on Oregon Country.  Britain and United States had meanwhile agreed to occupy the region together.  Many Americans began settling there and insisted Oregon become part of the United States.  In 1846 Britain and the United States signed a treaty giving us all of Oregon land.

 By March 1845 Congress had already approved annexation and needed only the support of the Republic of Texas.  Supporters believed that if Texas joined the States it would solve the republics financial and military problems.  Texas became a State in December  This action angered the Mexican Government, which considered Texas a stolen province.

 Mexico reacted to the annexation of Texas by cutting of all diplomatic relations with the United States.  They demanded the removal of all American settlers in California and banned immigration there.  In June 1845 Polk ordered General Zachary Taylor to take U.S. troops in the border region.