How were borders patrolled in history? They seem too large to patrol

Think of the Roman Limes or the Chinese Great Wall. There were no organized groups of millions of border-crossers at the time. And if there were disorganized groups that strong, empires fell. Even 50,000 troops would be an enormous army, fielded only by the largest states.

The goal was not to prevent individual traders (or migrants) from crossing. The goal was to prevent raids from "barbarian" tribes beyond the border. A good raid consists of getting in, getting the loot, and getting out again with the loot. The border fortifications served as early warning when "barbarians" got in and as bases for mobile military forces who would cut their retreat.

Re. the Iron Curtain, East Germany had around 50000 border troops for around 1500 kilometers of border to West Germany and to West Berlin. That is about 30 soldiers per kilometer of border or (statistically) one soldier for every 35 meters of the border.

In practice the East German border troops were primarily supposed to stop individuals from leaving East Germany, and together with two fences or walls and a clear strip of land between them, watchtowers, trenches (to stop vehicles), hightened security in areas close to the border etc. these 30 soldiers per kilometer were enough for that purpose, at least most of the time.

Larger groups of people crossing from the west to the East would either be dealt with by the police (in case of civilians) or the army (in case of an invasion). Re. crossings from east to west, the Stasi would have prevented the formation of groups large enough to simply overwhelm the border defences.

Just to supply a piece that is hitting pretty much every answer, but isn't being said, the idea that it was feasible, reasonable, or desirable to guard the entire length of every border against individuals hoping to relocate by sneaking over through the wilderness is a very modern concept. For most of world history, this simply wasn't something governments cared much about.

In general what rulers traditionally felt they could control was the transit of goods, particularly for the purposes of gathering taxes on trade. This is much easier, as any serious transit of trade is going to need to stick to roads, rivers, and ports, all of which are much easier to post a few guards on.

Guarding against military raids is also much more feasible by posting soldiers at these same points. Anyone trying to sneak a large force off-road is going to go much slower, and the same transit systems can be used to muster forces to counter them quickly.

There are generally only 3 reasons a ruler may want direct control over the migration of civilians:

• To preserve the existing "National" character of the country against other ethnicities (racist reasons).
• To prevent valuable human capital from leaving the country (prison states).
• To keep the numbers of destitute refugees down to a level the receiving country can humanely supply (due to massive forced displacements)

Of those 3, the first wasn't something a government would care much about before the advent of Nationalisim in the late 18th century.

The second was mostly only used for people with specific skills or knowledge, like trade masters for mills or finished good production. This didn't become a general emigration prohibition with the full length of borders guarded prior to the advent of prison states in the mid 20th Century, like the USSR, GDR, and PRK. These are countries that kept a highly disproportionate amount of standing military and police staffing, for just this reason.

The third has always been with us, but due to the large numbers involved can mostly be shut down greatly by controlling the traditional points of entry in more or less the traditional way. It also tends to be a temporary migration event, rather than a permanent state of affairs. Eventually you'll run out of people to force out.

The USA for instance didn't start trying to control its immigration directly until 1875, and then it was for pretty much entirely racist purposes, as large numbers of Chinese had begun to immigrate. For the first century of its existence, the USA got along happily (in fact, thrived) without any direct immigration prohibitions.

First, define what you mean by "history". There's quite a lot of it, you know :-)

With few exceptions (like Hadrian's Wall or the Great Wall of China, where the people on the other side were fairly aggressive and inclined to invade), borders simply weren't patrolled. The idea that individuals should be stopped simply didn't occur to most people.

WRT to Canada and Mexico, for most of US history it was possible to simply walk across - or ride horses, paddle canoes, &c depending on the terrain. Even as late as the 1980s, crossing into Canada via road at an official border crossing was a simple matter of "Welcome to Canada, enjoy your visit".

There are 3 major types of Border Controls:

• Immigration Control
  • the control of persons crossing a border
• Customs Control
  • the control and taxing of goods crossing a border
• Military fortification
  • the prevention of an invasion

In Europe, from 1850's until August 1914, border Controls were primarily Customs checks.

With the exception of Russia, passports were generally not required to cross a national border.

The control of 'strangers' inside a country by local authorities was common place, but was not primarily aimed towards foreigners but more to criminal (or political) elements independent of nationality.

In times of military buildups, troop consentrasions tended to be well behind a border and not at the border itself. When military mobilisations started, they often needed a few weeks to get up and going.

So any patrolling at the borders was mainy done to catch suggelers. There was less need for a sealing off of a border as you often have now.

All of this changed during World War I.

Passports (with photos) became manditory. Visas and work permits became common place. Military stratagies changed with border fortifications becoming common place during peace times, that in the end didn't bring the desired result.

So during this time, border Controls started to be a combination of Immigration and Customs.

After World War II, border fortifications served an additional purpose in enforcing the separation of societies.

As to the Iron Curtian, this was indeed a gigantic effort, between the early 1950's to the end of the 1980's, to systematically seal off their border preventing any crossing without their approval. The cost, in resources and peaple, was enormous and in the end also didn't bring about the desired result.