Final answer:
The nation that built the irrigation system that caused the desertification of the Aral Sea is Russia.
Step-by-step explanation:
The nation that built the irrigation system that caused the desertification of the Aral Sea is Russia. In the early 1960s, the Soviet Union diverted the Amu Darya and Syr Darya Rivers for irrigation purposes, resulting in the disappearance of the Aral Sea. This action was driven by the Soviet Union's desire to increase cotton production and make it a major export.