Excerpts from Books and Wikipedia
"In 1837, a proposed annexation was rejected by President Martin Van Buren, who feared war with Mexico over the issue. . . . The annexation was ratified by Congress in December, making Texas the 28th state (and included parts of New Mexico and Colorado). . . . The following spring, President James K. Polk invaded a portion of Texas still claimed by Mexico, thus initiating the Mexican-American War, which ended in 1848 with further land transfers from Mexico to the United States." [Furtado: 1001 Days] "Mexico did not recognize Texas independence, however, and so when Texas became a US state in 1845, fighting soon erupted again. Mexicans feared US desires to acquire more Mexican land, especially California, but Mexico was too weak to defend itself against the United States." [Born in Blood & Fire: A Concise History of Latin America, 4th Ed., p. 159]