For as long as some have had plenty and others little, there have been those willing to use force to appropriate the property of others. Throughout the world, Governments have evolved to (among other things) enforce property rights. Out in the open ocean, flying a flag of convenience, a seasteader must take care of themselves and their own.
The appropriate method of defense depends hugely on what threats a seastead faces: Mutiny, Piracy and Fraud may all be considered threats, but must be dealt with in completely different ways.
Many issues surrounding threats and a seastead's response to them depend on the politics and ethic of the seastead in question. A pacifistic seastead would react in completely different ways to piracy and mutiny than many others. This article will not, therefore, attempt to adjudicate on "right" and "wrong" responses to various threats.
Procurement of arms may also present difficulties, and an exploration into the legal issues surrounding large-scale arms purchases needs to be explored.
The seasteads will best defend themselves through a combination of methods, including physical arms, evasion, trade, and good relations. Surveillance technology as well as open relations with citizens on land will enable the seasteaders to spot potential dangers, and mobility will enable them to evade these dangers. Individual arms, defense weapons and critical mass will deter small-scale aggressions. An emphasis on trade and commerce will make nations view seasteaders as a valuable neighbor, and will probably go the farthest toward ensuring safety from what represents the greatest threat to seasteaders: government aggression.
(infectious diseases, vermin infestation, etc)
Potential external threats can be broken down into two main categories: governmental vs. non-governmental. While the governmental threats are far more dangerous, both categories need to be addressed, especially for the layperson who is apt to focus more on pirates and small-scale criminals. Threats may also be divided into internal v. external, with internal threats including common criminals who make their way on board, but the emphasis here will be on avoiding organized large-scale attacks.
Established government threats include the primary world powers: US, Russia, China, and arguably Britain and France. Also of concern is the looming possibility of world government, e.g., the growing jurisdiction of the UN. The UN Law of the Sea Treaty  may be of the most relevance: should it be ratified, any manmade island or structure is to be under the jurisdiction of the nation in which it was built, or the closest nation to its location. The fledgling seasteads would be no match for the conventional armies, navies and Air Forces of the established governments: they have superior firepower, could destroy infrastructure such as water purification facilities, greenhouses and agriculture plots, and could impose blockades and embargoes. Even if they do not directly attack, they might cut off bank accounts, credit lines and otherwise hamper economic freedom.
With pirates and terrorists, as with established armies, there is the threat of murder, rape, theft, human trafficking and enslavement. It is also conceivable that the pirates could attack supply boats even if the seasteads themselves are sufficiently defended. In reality, however, the threat of piracy is largely exaggerated. Most piracy occurs on a small scale: they are the equivalent of muggers at sea, and a large, people-dense seastead will not be an appealing target. A concrete-based seastead will be far harder to surmount for the average pirate than even the typical luxury cruise ship, which rarely comes under pirate attack. Large-scale piracy usually involves cargo ships, with a high ratio of cargo to crew; again, the typical seastead should not be tempting.
The resentment of ordinary citizens may also affect the seasteads, with the perception that seasteaders are merely fleeing their responsibility for paying taxes and “contributing to the common good.” Negative perceptions by the public may embolden governments to attack or otherwise use force against seasteaders. These same citizens, however, present just as great a threat to the liberty of those they coexist with on land; they are an argument for, not against, the improved freedom afforded by seasteading.
Openness and Non-Aggression
Avoid initiating aggression and do everything possible to appear open and non-threatening. General peacefulness, with a reliance on trade, diplomacy, and good public relations, will be greatly useful. A focus on openness, military transparency, and good publicity will go far in staving off military or economic assault. Governments may not want the bad PR of attacking peaceful seasteads that contribute value to the world economy.
Extreme mobility will probably prove the seasteaders’ most effective physical defense. Seasteads that are particularly concerned about safety may choose to take the form of ships for enhanced getaway ability. If the seasteads are able to move quickly enough, they may escape before governments or other disgruntled groups are able to mobilize for attack. The seasteads can also reduce the amount of stuff physically on board to keep mobility high and to reduce the temptation to pirates. Many commercial shipping companies already rely on evasion and mobility to escape threats, rather than spend money on arms and crew training. This extreme mobility would mean that seasteads frequently stop in ports around the world to replenish supplies. The seasteads would be less independent; the high volume of trade may mean that the seasteaders would establish positive relationships with friendly nations, but it can be difficult to predict sailing/arrival times, and resources are not uniformly available around the globe.
Self-Sufficiency and Trade
Instead of, or in addition to, extreme mobility, seasteads can develop greater self-sufficiency. Food could be grown on platforms and the surplus sold or traded. Seasteads would not have to rely on land-based nations for food and supplies, which would be useful if nations or world government became hostile. In addition, it may be feasible to increase the amount of stuff on board because greater wealth means that seasteads are better able to defend themselves. Seasteaders, however, may find a way to store wealth off board or electronically, without resorting to weighing down the platforms.
Passive resistance is yet another option if evasion methods fail, since it is unlikely that the seasteads could resist a large-scale military invasion. But emphasis on being good neighbors with valuable services to offer should stave off such a catastrophic scenario.
On the community level, separate or independent seasteads could agree to band together for defense, presenting a critical mass to would-be attackers.
Deterrence could involve hiring a private defense agency in combination with or funded by an aggression insurer to safeguard against or retaliate against aggressors. Seasteaders could contract with established land-based armies and defense groups to rent fully-crewed defense vessels, especially in the initial phases of seasteading. Presumably, arrangements could be worked out for payment to be made only after a successful counterattack were launched.
Reliance on original citizenship, on the idea that governments would respect and come to the rescue of its citizens, may prove a double-edged sword. It is difficult to predict how reliably governments would offer aid even to citizens who still paid taxes, and to some it would appear to subvert the whole purpose of seasteading. In the worse-case scenario, governments may capture or murder their seasteading citizens under the pretense that they are lawless terrorists, or because they are seen as tax evaders.
The greater efficiency of the seasteads versus that of conventional state defense budgets should enable the seasteaders to obtain an impressive arms outlay with less cost.
(Note that many "less-than-lethal" weapons can be lethal under some conditions. Many people have been killed by Tasers for example, and many baton or unarmed strikes are potentially lethal.)
Microwave emitters - Any directed energy weapon is potentially as dangerous to the operator as the intended target. A reflection from an energy weapon is equivalent to a ricochet from a bullet, but probably more dangerous since less energy is dissipated.
Sound wave emitters
Striking weapons - nunchucks, batons, etc. All require specialized training and some are more practical and/or effective than others.
Incapacitants - Pepper spray, etc. Pepper spray, tear gas, etc., are relatively ineffective against people who have trained against them, i.e, those who have gone through military training.
Personal defense small arms
Pistols Semiautomatic, .40 S&W / 10mm / .45ACP class
Shotguns - 12 Ga Pump / Semi / Full variety of ammunition including beanbag, rubber ball
Individual small arms
Rifles- Preferably select fire assault rifles, make and model being dependent on budget. Reliability will be important because it is unlikely that there will be a gunsmith in the early days of seasteading.
Crew-served small arms
Area defense weapons
CIWS Air and close-in point defense Man-portable air defense (MANPAD) - Stinger, SA7, etc., anti-aircraft missiles
Intermediate range, air defense
Sea Sparrow (type)
Intermediate range surface defense
Cruise missiles such as Exocet (type)
Surrounding small boats full of explosives could also be used to deter would-be attackers. No modern navy will allow small boats to approach.
Structural defense mechanisms
Partial or complete flooding of selected areas, up to the reversible submersibility of the entire structure
Heavy Arms Drawbacks
A drawback of the arms approach is that it may hurt public relations: governments may be provoked by a heavy arms buildup, while citizens and governments alike may perceive the seasteaders as terrorists. For this reason, as well as issues of cost and scale, the seasteads should not rely on very large-scale arms buildup.
Threat Surveillance and Detection
Above- and Underwater Cameras
Area Radar Aerostat radar (type)
Active / passive sonar?
Individual Equipment and Protection
In sea as on land, perhaps the best primary defense is to arm each man, woman, and child with personal firearms, as well as unconventional personal defense equipment such as tasers and batons. Even the appearance of being well-armed should discourage attack, especially from internal criminals and pirates.
Fire resistant clothing (shirt/trousers or jump/flight suit), gloves, head covering (balaclava), boots
Load carrying equipment
Vest with integrated pouches for ammo, water, first aid kit, etc. With armor and/or flotation
Night vision devices
Helmet with face shield, integrated communications and hearing protection
EMP is not practical. See Wikipedia.
directed energy weapons
Lasers are somewhat practical.
metalstorm / flechettes
in general, use the stuff that the big militaries paid to have developed, and then couldn't use because of the bureaucracy. see steyr ACR for an example.
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