How were the Americans justified for the acquisitions of Texas and Mexico between 1845 and 1848?

It is a matter of perspective whether the United States was justified in taking Texas and other lands from Mexico in the 1840s. The Mexican government never officially recognized Texas as an independent country, and they did not think the United States should annex Texas. The Mexicans also felt the United States illegally moved its military into Mexican land between the Nueces and Rio Grande Rivers. Therefore, the Mexicans did not feel the United States was justified in the actions that led to the Mexican-American War and, eventually, the Mexican Cession.
From the American perspective, the United States felt it should expand from the Atlantic Ocean to the Pacific Ocean. The Americans knew that the people of Texas wanted to join the United States. Many Americans had gone to live in Texas and wanted to be part of the United States. Therefore, the United States felt justified in annexing Texas.
The United States also was interested in gaining land from Mexico. The Americans wanted to spread their culture and way of life to new lands. The Americans also wanted to settle lands west of the Mississippi River. The Americans believed that by expanding to lands controlled by Mexico, the Americans would be able to more quickly settle and develop these areas. Thus, the Americans believed, in the name of progress, that it was acceptable and justifiable to settle these areas that Mexico had controlled.
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