The Mexican American War - A detailed thesis of the origins, events and outcomes of the US-Mexican war, sourced largely from the diary of President James Polk. If you can get past the anti-American bias and rhetoric, this site actually has some good information.
From the site:
Polk was a confirmed expansionist; he was elected on the platform of bringing both Oregon and Texas into the union. He asserted that he received a mandate from the electorate, all be it by a small margin, to proceed with his antagonizing of the Mexicans. It was certainly Polk who initiated the commencement of the war, at least in terms of actual confrontation. Polk sent a contingent of American troops under the command of General Zachary Taylor into the now annexed Texas to guarantee its sovereignty. When Polk ordered Taylor into the disputed Rio Grande area, he effectively insured a war by stationing American troops on what the Mexicans held to their sovereign territory. Polk further encouraged the Mexicans to commence open conflict by leaving Taylor with precious few men in the face of a significantly larger Mexican force, inciting the Mexicans to try their hand against the Americans.