Union and Confederate forces first met at ______________.

The first significant encounter between Union and Confederate forces in New Mexico occurred at Valverde, which is why this answer is correct. This battle took place on February 20, 1862, during the American Civil War as part of the larger New Mexico Campaign. Union forces, under the command of Colonel Edward Canby, faced off against Confederate troops led by Brigadier General Henry H. Sibley. The battle was important in the context of the Civil War as it highlighted the strategic interest of both sides in the Southwestern territories, particularly due to the region's resources and trade routes.

The other locations mentioned do not reflect the initial confrontation between Union and Confederate forces in this area. San Antonio is primarily associated with Confederate logistics rather than battle. Santa Fe was a primary target during the campaign, but it was not where the forces first clashed. Glorieta, although a significant battle later in the campaign, occurred after Valverde and was part of the Union’s efforts to counteract the Confederate advance in New Mexico, making it a subsequent engagement rather than the first.