Respuesta :
The justification relies mainly on geopolitical strategy. Since the power of the Mexican army to defend such vast and rich territories was very weak, the Mexican government was considering selling some states to foreign powers in order to pay foreign debt. England and France had their eyes upon Upper California, which was probably the richest state in Northern Mexico. In this case, these European powers would become a threat to the indisputable authority of the US in North America. To avoid that possible panorama, the US started a war against Mexico to claim dominion on these territories. On the other hand, the US didn't want to lose the opportunity of taking these lands and having a strategical access to the Pacific Ocean.
The justifiability of the U.S expansion was questionable due to the economic shortfall midway, the political gains made, and the damage to other societies that lived in the areas. The economic shortfall that occured midway was because the lack of available farmland, as then expanded and settled, more and more resources they could not get themselves had to be imported in while farming was not as high quality as in the east due to the worse soil. The political gains made during the expansion were also questionable. Politicians kept referencing "Manifest Destiny", equivalent to some buzzwords today. This caused an obsession with expansion that ended up collapsing under its own weight. The third issue was the incredible loss to societies that lived in the western areas, foreign diseases wiped out swathes of native populations, and slowly treaties that were supposed to be upheld were broken as the remaining natives were pushed into smaller and smaller areas. In conclusion the expansion was questionable because of the economic shortfall, the obvious political games, and the incredible damage inflicted on the natives.