The document discusses the territorial expansion of the United States between 1800-1850, including the annexation of Texas and war with Mexico. It outlines how Texas gained independence from Mexico in 1836 and became its own republic before joining the US in 1845. Disputes over the border between Texas and Mexico led President Polk to send an envoy to negotiate, but when Mexico refused, war broke out between the US and Mexico from 1846-1847, resulting in the US acquiring vast new western territories of California and New Mexico.