The United States in 1846 was not justified in going to war with Mexico. The United States did not have proper justification to respond with violence against the Mexican government. The war with Mexico was also a product of the United States' belief of manifest destiny. Polk's over ambition to seize new territory from the Mexicans and disappointment over their refusal to sell him California also possibly played a factor in his willingness to wage war against Mexico. The United States under the leadership of president Polk clearly provoked Mexico into attacking US troops. All these reasons show that the US had no business starting a war with Mexico for territory that was rightfully theirs.
The war with Mexico came at a time when much of the country had strong feelings of manifest destiny. Manifest destiny is the belief that fate had preordained the US to expand from the Atlantic to the Pacific oceans and from Canada to the Rio Grande river. This idea which was coined by John O' Sullivan was very popular in the 1840's. This ideal had strong influence and was one reason that their was so much popular support for the US expansion west. This ideal while nationalistic did not give us the right to go into Mexico and seize land which was rightfully theirs in the first place. They had the right to expel any US citizens that were living on their country's land especially if they where not abiding by their laws. This alone makes even the annexation of Texas not completely just. The US vision of manifest destiny helped to win the public's support for the Mexican war. Yet the public was misled by this sense of manifest destiny and those that did support it supported an unjust war.
President Polk during his presidency lusted for more land than the country had ever before controlled. Not only did he capture Texas but also California and Oregon. While attempting to gain California through peaceful means he sent John Slidell to