Final answer:
The U.S. placed troops along the Texas frontier due to a boundary dispute with Mexico. Expansionist desires led to conflict, and President Polk used a skirmish between U.S. and Mexican troops to declare the Mexican-American War. Tensions along the border continued to influence foreign policy actions such as the Zimmermann Telegram during World War I.
Step-by-step explanation:
The United States placed troops along the Texas frontier due to ongoing tensions and disputes over the true boundary between the U.S. and Mexico. After annexing Texas in 1845, the U.S. claimed the Rio Grande as the border, while Mexico argued for the Nueces River as the established boundary. With expansionist desires pushing U.S. policy, President Polk aimed to extend the nation's territory. When diplomacy failed, and a skirmish between U.S. and Mexican soldiers occurred, Polk used the events to justify a declaration of war, marking the beginning of the Mexican-American War in 1846. Additionally, prior tensions, including the interference in Veracruz in 1914 and the subsequent pursuit of Francisco "Pancho" Villa, contributed to the U.S. military presence along the border, which raised concerns of potential Mexican invasion.
The annexation of Texas and the desire to extend U.S. borders not only led to a war with Mexico but also played a part in the broader geopolitical maneuverings seen in the Zimmermann Telegram during World War I. This broader context highlights the continuum of border security concerns and foreign policy implications extending far beyond the initial conflict.