It was the nationalist centralists who demanded war with the USA (who at the same time, also wanted war with Mexico) in 1845.

A centralist mexican dictatorship was overthrown in Mexico in 1845; federalist/liberal José Joaquín Herrera became president. It was at this time that Texas was annexed by the United States. The centralists wanted war due to this; Herrera wanted to negotiate.


Mexican federalist José Joaquín Herrera

The fact that Herrera did not declare war infuriated the centralists who immediately attempted to overthrow Herrera with success. Herrera was to negotiate with american government representative John Slidell; when Slidell arrived, Herrera under pressure from conservatives refused to meet with Slidell and later was betrayed and overthrown by General Mariano Paredes and Slidell was kicked out of the country.

American General Zachary Taylor was ordered to move to the Río Grande shortly after this (the Río Grande was never agreed as a border by the mexican government; therefore, this effectively was an unauthorized invasion of what the mexican government considered its territory) and Mariano Paredes issued a declaration war and attacked Taylor, thus starting the Mexican American War.

The conservatives were not fueled by nationalism; they believed they could win the war. And in fact, they could. The Mexican Army was at least 4 times larger than the by then American Army, was well armed, had experience and was well trained. If it wasn't for the effectiveness of american light artillery the USA had a good chance of being defeated by the mexicans.

And thus, the mexicans suffered truly embarassing defeats (See: Battle of Churubusco) which effectively ended in the loss of half of the mexican territory. Although, the war wasn't certainly cost-free for America; 13,000+ american soldiers were killed in the conflict. About twice the number of mexican soldiers (around 25,000) lost their lives as well.

http://en.wikipedia.org/wiki/Jos%C3%A9_Joaqu%C3%ADn_de_Herrera

http://www.lone-star.net/mall/texasinfo/mexicow.htm

You spelt Zachary wrong.

Actually since Texas beileved the Rio Grande to be there border and are reason for involvement was for the defense of are kin in Texas we were literally defending their claims and at was beileved to be the border.

Texas was annexed in 46 not 45. Under texan desicion as joining the Union was a popular notion in Texas, they tried several years earlier during the Tyler Administration but the US did not seek war so they denied it.

Butthurt above ^

Okay then gotcha TG, big misunderstanding.

It was annexed in 45, became a state in 46.