How does the US have a justified in declaring war against Mexico?

The U.S. justified its declaration of war against Mexico in 1846 primarily through claims of defense and territorial expansion. The immediate trigger was the disputed territory between the Rio Grande and Nueces River, following the annexation of Texas, which Mexico still considered its own. The U.S. also cited the killing of American soldiers on what it claimed was its soil as a reason for military action. Additionally, there was a prevailing belief in Manifest Destiny, the idea that the U.S. was destined to expand across North America, which further supported the war effort.