New Mexico was most valuable to both the Union and the Confederacy because ______________

New Mexico held significant strategic value during the Civil War primarily due to its location, which provided critical trade access to both the west coast of the United States and to Mexico. This trade route would have facilitated the movement of goods, resources, and military supplies, making it a key asset for both the Union and the Confederacy. The ability to control trade routes was essential during the war, as it allowed for the distribution and acquisition of necessary provisions and materials.

The other options, while they might highlight certain positive aspects of New Mexico, do not capture the primary reason for its value during the Civil War. The notion of fertile farmland, for example, is not a primary factor in the region's strategic significance compared to its trade routes. Similarly, the economic standing relative to California or the appeal of being referred to as the 'Land of Enchantment' does not directly link to military strategy and control during the conflict. The emphasis on trade access accurately reflects the situational context of the era, making it the correct choice.