Final answer:
The attempt by President James Polk to purchase California and New Mexico was unsuccessful, leading to conflict and the outbreak of the Mexican-American War. The Treaty of Guadalupe Hidalgo subsequently resulted in the U.S. acquisition of significant Mexican territories.
Step-by-step explanation:
When President James Polk tried to purchase California and New Mexico and secure the Rio Grande as the Texas border, the attempt was not successful, resulting in a diplomatic breakdown and escalation to conflict. The situation led to the Mexican-American War. Despite the intention to acquire these territories peacefully and the offer to pay $5 million for New Mexico and up to $25 million for Alta California,
the Mexican government was outraged by the proposal and the infringement on their sovereignty, and they responded by cutting diplomatic ties with the United States. The Mexican-American War erupted, culminating in the U.S. invasion of Mexican territory and the eventual Treaty of Guadalupe Hidalgo, where the U.S. gained control of a vast amount of Mexican territory, including California, Arizona, New Mexico, Texas, Utah, Nevada, Colorado, Wyoming, and Kansas. Therefore, the correct answer is B: The purchase was unsuccessful, leading to conflict and the Mexican-American War.