The Mexican American War - How Did it Start?

The Texas Revolution gave independence to the Republic of Texas. However, it also sewed the seeds that would grow into the Mexican American War. Texas immediately seeks annexation from the United States, however, the USA denies Texas. The Republic of Texas clings on to it's sovereignty for nine long years until it joins the Union in 1845. When James K. Polk takes office, he immediately sends Zachary Taylor to enforce the entire boundary claimed by the Republic of Texas at the Rio Grande River. Taylor marches to the Rio Grande, igniting the chain of events that start the Mexican American War.