Final answer:
The United States came into conflict with Mexico as a result of the annexation of the Republic of Texas, which led to the Mexican-American War and significant territorial losses for Mexico.
Step-by-step explanation:
The conflict that arose between the United States and a foreign nation due to the annexation of the Republic of Texas was with Mexico. The annexation led directly to the Mexican-American War, which was precipitated by the disputed territory and borders. The United States' annexation of Texas in 1845 was seen as an act of aggression by Mexico, as they did not recognize Texas's independence and claimed the territory as their own. As a result of the war that followed, Mexico lost a significant amount of territory to the United States, which included many of the southwestern states we know today. The Treaty of Guadalupe Hidalgo, which ended the Mexican-American War in 1848, forced Mexico to cede these lands to the United States and set the Rio Grande as the boundary.