Final answer:
On the eve of the U.S. invasion of Mexico, military conflict best describes the situation due to heightened economic conflict, territorial expansion motives, and failed diplomatic efforts between the United States and Mexico.
Step-by-step explanation:
On the eve of the U.S. invasion of Mexico, the situation best described what was happening is c) Military conflict. Economic conflict and military tensions marked the relationship between the United States and Mexico, which were heightened by a series of incidents leading up to the Mexican-American War. The annexation of Texas and subsequent border disputes were particularly contentious issues. Diplomatic efforts like the mission of John Slidell to avoid war by resolving the border dispute and attempting to purchase New Mexican and Californian territories ultimately failed, as was evident from Mexico's reaction to annexation by severing diplomatic relations and both nations preparing for war.
The incident at Veracruz and the Zimmerman Telegram incident are examples of the military conflicts that further inflamed U.S.-Mexico relations, leading up to the Mexican-American War. This period in history was characterized by territorial expansion, and the desire to maintain a balance in free and slaveholding states played a significant role in the conflict that unfolded, eventually leading to the Mexican Cession of 1848 and subsequent debates about slavery in the annexed territories.