History is littered with examples of rulers building walls to keep out undesirables, invaders and immigrants. The Great Wall of China and Hadrian's Wall in Britain are among the best-known examples, and there is a 2,000-mile (3,200km) fence between India and Bangladesh.
The earliest-known barrier for this purpose was in the cradle of civilisation in Mesopotamia about 4,300 years ago. A 110-mile wall called the "Repeller of the Amorites" was built to keep out the tribe of that name who were known as "a ravaging people with the instincts of a beast".
This was because they did not grow grain but hunted for their food. The wall did not work, however, because the Akkadian empire was being overwhelmed by a force greater than the Amorites - climate change. A drought in the region that lasted for 300 years destroyed the empire. During that period, the whole Middle East region was subject to mass migrations as various tribes looked for greener pastures as their homelands dried up.
Exactly what caused that drought is still not clear, but the effects are exactly those that are beginning to happen worldwide at the moment - an unpredictable climate forcing millions of people to migrate to survive. History teaches us walls will not solve the problem.