If the Mexican War had not occurred, many things would be different. For example, 525,000 square miles of new territory may not have been acquired. The United States would then be without California, Nevada, Utah, Arizona, and New Mexico. Relations between the United States and Mexico would also not be tense. Most importantly, however, is that the expansion of slavery in the newly acquired Mexican territories became the major constitutional and political issue that led to the Civil War. Therefore if it had not occurred, the American Civil War might not have, either.
"No action comes without a consequence." Several of America's actions ignited the fire of the Mexican and American tension, however many forces could have stopped the War before it started. On the last day of Tyler's term in office, he created a joint resolution of Congress which would have required a two-thirds majority that he did not have. On the last day of his term, Tyler sent messengers to Texas for the purpose of immediate annexation. The outcome of those events could have been avoided by Polk, the new president, if he would recall the messengers and agree to negotiate a new treaty.
Then, Mexico would be pacified. Also, if the Americans did not have such a strong desire to move westward the Mexican War may not have ever occurred.
Westward expansion was completely inevitable. The West provided people with new opportunities. For example fur traders had so much competition between themselves that they had to move further west in order to find a new way of life. Also, there was