There were several reasons why the United States was justified in expanding into Mexican Territory in 1846.  The United States and Mexico had differences over the United States-Mexican border in Texas.  Mexico insisted the border was further north than we said it was. We believed the border was at the Rio Grande River while Mexico believed the border was at the Nueces River. We sent a representative, John Slidell, to discuss this with the Mexican leaders.  However, the Mexican leaders refused to meet with us.  We were prepared to offer Mexico a large sum of money ($30 million) if they were to accept the Rio Grande River as the border and give us California and New Mexico.  Since the Mexican leaders wouldn’t meet with us, that offer was not presented to the Mexican government.  Finally, when American soldiers were attacked in the disputed territory, we went to war against Mexico.  There are several reasons why the United States felt justified in expanding into Mexican territory.