Agua Caliente in Maricopa County, Arizona on the border with Yuma County, is a place north of the Gila River near Hyder, Arizona. Named 'Santa Maria del Agua Caliente' in 1744 by Father Jacob Sedelmayer. In 1775, Father Francisco Garces used the current short form. The location was the site of a resort established at the site of nearby hot springs. Agua Caliente, a name derived from Spanish meaning "hot water", received its name from nearby hot springs which were originally used by the local Indigenous population.

Camp Hyder was a US Army installation in Arizona, functioning as a subcamp of the Desert Training Center in Riverside County, California. The main headquarters for the Desert Training Center was Camp Young, this is where General Patton's 3rd Armored Division was stationed. Camp Hyder is 2 miles (3.2 km) miles south of Hyder, Arizona. The camp was just north of the Gila River. Camp Hyder is 60 miles (97 km) miles east of Yuma, Arizona, near Camp Horn. Camp Hyder was built at the site of an old 1890s military base. Trained at Camp Hyder, in 1943, for six months was the 77th Infantry Division from April 1943 to September 1943. Then the 104th Infantry Division moved in for training. Unlike the other camps, no large tank activity was done. The camp was built by The 369th Engineer Regiment. Camp Hyder had its own rail station at which most troops arrived. The train station at Sentinel, Arizona south of the camp was also used. Over 13,000 troops were trained at Camp Horn and Camp Hyder.