I'm curious what you think on this issue. I used to view the Mexican American war as a justified war by the United States until I began to study US history. Now that I know what the political atmosphere was (Manifest Destiny and all that) was during the time I'm kinda disgusted at the fact that the U.S. was involved in this war. I kinda see some similarities with the Gulf War and the Mexican American War. The U.S. wanted more land just as the Iraqis and they initiated a war with a less powerful country in order to take said land. Do you think this opinion is valid? I'd like to hear some opposing views.
Just regarding your question as to whether the war was Mexican Aggression...the answer is either an unambiguous no or a confused, qualified, and highly technical yes, depending on what perspective you approach it with.
The proximate cause of the war was the annexation of the Republic of Texas by the United States in 1845. The Republic of Texas was a breakaway country of mostly Anglo settlers who had migrated (especially from southern states like Tennessee) into Texas and settled there, yet grew disenchanted with the Mexican government's centralizing dictatorial policies (in particular, Mexico's opposition to slavery and promotion of Catholicism, but more generally, the ending of regional autonomy), and rebelled and formed their own country. It's critical to point out that Texas was not the only breakaway country from Mexico in this time period...many regions attempted to set up their own country, most notably the Republic of the Rio Grande (northeastern Mexico/southern Texas today) and the Republic of the Yucatan (the Yucatan peninsula). Texas was the only one of those states that broke away over the true long-term, although the Yucatan Republic lasted 1841-1848 and there was some reasonable expectation that Mexico would continue to fragment.
Texas and Mexico, as you can see in this map, had very different views over what the appropriate boundaries of Texas were, and, in fact, Texas' second President, Mirabeau Lamar, sent a humiliatingly failed military expedition to press Texas' claims on Santa Fe in 1841. The Republic of Texas, a borderline failed state with no sound economic footing, applied for US annexation in the mid-1840s and proved a major campaign issue in the 1844 election, with both candidates aware that annexation of Texas could provoke war with Mexico. Polk enthusiastically backed annexation of Texas, Clay hesitantly and with reservation endorsed the annexation of Texas. Polk won and John Tyler organized the annexation of Texas in his lame duck period.
Part of the problem that emerged: what exactly had the United States annexed? Did it annex the territory that the Texans had claimed or did they annex just the parts of Texas that Mexico recognized as Texas? Did Texas end at the Nueces or the Rio Grande river? Polk sought to force the issue, and sent Zachary Taylor to set up camp in the disputed territory, south of the Nueces but north of the Rio Grande. The provocation accomplished what it intended to, and when the Mexican army defended the land they claimed, Polk had his excuse to wage a full war with Mexico.
Was it unwise for the Mexican army to begin hostilities with Taylor's force? Possibly, but what other option does a country have when their neighbor sends an expeditionary force to occupy disputed border territory? Does the Mexican action sound like an act of aggression?
Polk characterized it thusly:
"The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte. But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced, and that the two nations are now at war."
Was that American blood shed on American soil, or on Mexican soil?
Gama_Rex has a good answer. But though the War did happen, there were still voices against it, at the time and later. Henry David Thoreau refused to pay his poll taxes, because of his opposition to the War, and was put into jail for it ( which resulted in the famous interchange with R.W Emerson, when Emerson asked, "What are you doing in there?" Thoreau asked, "Waldo, the question is, what are you doing out there?")
You will also find the opinion by Ulysses S Grant in his Memoirs, that the war was simply a large and powerful country bullying a smaller one. This is perhaps understandable: Grant had distinguished himself in Mexico in the War, and though he had a poor view of the Mexican generals' abilities, he seems to have liked Mexico , and when he became president his administration was supportive of Benito Juarez.
