Analyzing Polk's Justifications for Expansion

In 1848, the United States experienced significant territorial expansion following the conclusion of the Mexican-American War. In the following excerpt from President James K. Polk's speech to Congress on December 7, 1848, he outlines his justifications for this expansion:

"Our country is destined to be the great nation of the world. With the acquisition of new territories, we not only fulfill our manifest destiny, but we also ensure that America remains a land of opportunity for all our citizens."

Using this primary source, analyze how Polk's speech reflects the key ideas of Manifest Destiny and the impact of such beliefs on American territorial expansion in the mid-19th century. Be sure to include specific references to the text as well as historical developments that support your analysis.