In March 1846, about 4,000 U.S. soldiers arrived at the Rio Grande. According to the U.S. claim, this river marked the southern border of Texas. But the area was actually controlled by Mexico. Mexico's leaders wereupset. They believed the United States had invaded their country. Building Our Nation is a series of AV2 media enhanced books. A unique book code printed on page 2 unlocks multimedia content. These books come alive with video, audio, weblinks, slideshows, activities, hands-on experiments, and much more.