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." [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]