If walls don't work...
Although Hadrian's biographer wrote "(Hadrian) was the first to build a wall 80 miles long to separate the Romans from the barbarians", reasons for the construction of the wall vary, and no recording of an exact explanation survives. Theories have been presented by historians, mostly of an expression of Roman power and Hadrian's policy of defence before expansion. On his accession to the throne in 117, Hadrian had been experiencing rebellion in Roman Britain and from the peoples of various conquered lands across the Empire, including Egypt, Judea, Libya and Mauritania.
These troubles may have influenced Hadrian's plan to construct the wall as well as his construction of limites in other areas of the Empire, but to what extent is unknown. Scholars disagree over how much of a threat the inhabitants of northern Britain really presented and whether there was any economic advantage in defending and garrisoning a fixed line of defences like the Wall, rather than conquering and annexing what has become the Scottish Lowlands and defending the territory with a loose arrangement of forts.
The limites of Rome were never expected to stop tribes from migrating or armies from invading, and while a frontier protected by a palisade or stone wall would help curb cattle-raiders and the incursions of other small groups, the economic viability of constructing and keeping guarded a wall 72 miles (116 km) long along a sparsely populated border to stop small-scale raiding is dubious.
Another possible explanation for the wall is the degree of control it would have provided over immigration, smuggling and customs. Limites did not strictly mark the boundaries of the empire: Roman power and influence often extended beyond the walls. People within and beyond the limes travelled through it each day when conducting business, and organised check-points like those offered by Hadrian's Wall provided good opportunities for taxation. With watch towers only a short distance from gateways in the limes, patrolling legionaries could have kept track of entering and exiting natives and Roman citizens alike, charging customs dues and checking for smuggling. Another theory is of a simpler variety—that Hadrian's Wall was partly constructed to reflect the power of Rome and was used as a political point by Hadrian. Once its construction was finished, it is thought to have been covered in plaster and then whitewashed: its shining surface reflected the sunlight and was visible for miles around.
The Great Wall of China, for instance, was built by the legendary (and legendarily cruel) Emperor Qin Shi Huang, the first true Emperor of China. Nobody really knows how long the original wall was, but it was rebuilt and restored and modified and lengthened repeatedly by subsequent Chinese Imperial dynasties and regimes. Apparently, the walls built under the Ming Dynasty - the last people to truly work on the thing - were up to 8,850Km, or 5,500 miles, in length (!!!).
And that is what the US military is for.