Marine MEU vs. Roman Legion
So, a thought on my mind that has been brought up is that, who would win in a fight; the Marine Expeditionary Unit or, the Roman legion? While it's difficult to exactly ascertain the exact figures, for these purposes I'm assuming 2,200 marines vs. about 100,000 or so Romans. If you need a backstory, marines have troubled back in time to try and give advanced technology to the Romans and take over their society so they can eventually build a new society and hopefully in the future prevent the oncoming disaster that will inevitability result from Trump being elected. It's humanities last hope! So, an MEU time travels and decides to take on the Roman Legion. Who would win?
At first this might seem random; why an MEU? What is an MEU? Are you on drugs? To answer all but the last question, a Marine Expeditionary Unit is a unit within the marines designed to be perfectly self sustainable. It's the smallest unit that can take care of itself; with it's own air fleet, tank crew, soldiers, special forces, logistics organization and command it can basically operate anywhere in the world on it's own initiative and serve as a small mini-army. In the case of Grenada, 10,000 marines were deployed to overtake the Island, which were similar to an MEU; when you need a small army to be deployed in a second's notice, these are the guys to call. They don't just fight conventional battles, they're capable of irregular and strange warfare, and operating completely by themselves without logistics supplies. They have their own water purification systems, trench diggers, the ability to establish military bases, hunt for food and even to an extent, produce ethanol and biofuels from nature. They are designed to remain almost indefinitely in the field, and have many years worth of supplies stocked up. These members are career soldiers and while they are not all among "elite" forces (I.E. special forces), they are far above average in terms of skill and ability. They expect to be on ships for months at a time or overseas for large sections of their career, making them psychologically prepared for a long stay away from home (in this case, for the rest of their lives).
The Roman legion on the other hand is perhaps the second best military of the time (after the Monghols). Their empire is the second largest in the world, and they have mastered the art of the "machine of war", able to systematically produce very consistent weapons and armor across their troops, trebuchets and catapults, and other such goods. They have involvement in Egypt with collections of some of the most valuable works of the ancient world, control the major supply trade networks of the day connecting the middle east, Africa and Asia with Europe (I.E. otherwise known as the Byzantine empire), and even things like paved roads across most of their empire, gigantic buildings and waterworks such as aqueducts which, like today, transport water across the empire. Many of these structures are still standing today, giving you an idea of their quality. They have copious amounts of food and water, tremendous amounts of wealth and even a semi-democratic system. For the time, they were nearly unmatched with the scope in which their technology and wealth was spread out among the average citizen, and their military was incredibly capable, as a result. At its peak, after the Antonine Plague of the 160s CE, it had a population of about 60 million and a population density of about 16 persons per square kilometer. In contrast to the European societies of the classical and medieval periods, Rome had unusually high urbanization rates. During the 2nd century CE, the city of Rome had more than one million inhabitants. No Western city would have as many again until the 19th century. Their military was 99% economy, having the best logistics in existence, and preferring long sieges where their men simply out lasted their enemies in long wars of attrition. There is a saying "all roads lead to Rome", as the Romans were able to cart goods across their empire and all the way up to the front lines with ease, remaining in combat indefinitely, without the romans having to carry their own food and water. For every Roman soldier there were likely 4 or more support troops behind them, giving them the ability to maintain their forces. The Roman legions were considered impregnable and invincible, as for as often as they lost men, they replaced them, recruiting them from the empire. They were seen an unstoppable force, moving like a glacier to consume everything around them.
The marines have the advantage of modern technology and knowledge. Not only do they have better weapons individually, and armor and other equipment, but also better knowledge. They've studied the Romans and the Battle of Cannae(the defeat of one of their largest armies), and thus know their strengths and weaknesses inside and out. They've also studied the Mongol Empire and other wars throughout history, giving them extensive knowledge of military tactics and strategies. Military colleges have built on these theories and have even more recent examples of tactics and strategies, such as WWII, Korea and the recent wars in the middle east, giving them far more complex knowledge. They fight a fast paced, almost Guerrilla style warfare, in urban environments which demand speed and agility. They're used to enemies that can kill them in a single hit, and have an emphasis on getting the first shot, since modern body armor will not stop rifle rounds and is aimed at stopping shrapnel. Comparatively, the Romans were much smaller and deliberate, with armor that took several hits to get through or at least a single direct hit rather than being so easily exposed and killed. They fought in tight defensive formations where as marines spread out, and marines also possess things like helicopters which can provide areal reconnaissance, night vision to fight the enemy when they're practically blind, the ability to rain fire and death from the skies, and even modern firearms, which far outrange and easily would pierce the armor of the Roman's.
The obvious advantage of the Romans is size and scope; they not only have more numbers than the marines, at 100,000 men compared to 2,000, but things like trebuchets and catapults which can large projectiles hundreds of yards. A 1 ton rock falling from the sky could presumably destroy a tank and decimate defense. They also had ballista and other such weaponry, allowing them to pepper the enemy with massive spear-like arrows raining down from the sky, which could legitimately pose a threat to modern marines. In addition ,they lived off the land, and possess the logistics to continue their operations. For the marines, it's a lot like a ticking clock; every action has a certain cost, from clearing wooded areas to digging trenches. There's a limit to their gasoline and resources, to their bullets and other supplies, and thus everything they expend is a lot like sinking further on a ship taking on water. If they have only 50,000 gallons of gasoline and they expend 5,000 of it digging trenches, there is a permanent loss to their supply even though we think of gasoline as being something we can refuel. There's a sinking ship, and when time runs out it will fall apart. The only question is will they achieve their goals by then? For all of our sake and humanities, we hope that the marine's win! So, who would win?
The Marine Expeditionary Unit
As stated before, the MEU was chosen for a reason. They are the smallest unit capable of functioning out in the field and being completely self sustainable, with their own aircraft, tanks, special forces, reconnaissance units and so on. They also have Marines possesses the advantage not only in raw firepower and technological innovation, but also in terms of strategy. 2000+ years of warfare to build on, on top of modern understanding of science and they can easily combat their enemies. They've not only studied battles such as those by Hannibal, Genghis Khan, Alexander the great, and other militaries, but those in more recent conflicts as well, such as WWII and the Iraq wars. They know what the stars are, that bacteria is not little gremlins crawling on their skins, are nowhere near as superstitious and uneducated as the Romans. For a morale factor, Romans attacked by the marines would be "awed" by their weaponry, and possibly believe it was the end times with hellfire literally raining down on them for the skies. While the Romans might in theory be able to outlast the marines, the fact of the matter is they wouldn't know if the marines would run out of their death machines or not. It's possible that they would surrender immediately after such a show of force and obey the marine's commands. With the marine "Shock and Awe" strategy already in use today, it would only be exemplified by the culture and technological shock the Romans would be experiencing. At the very least, many swaths of romans would throw down their arms and desert their forces as they see taking them on as a lost cause, and many more would only be lightly committed to combat.
Areal reconnaissance allows the marines to see what their enemies are doing from a birds eyes view. Reconnaissance in the old days not only took time, reports from second-hand accounts (I.E. someone seeing something and then describing it to you), but potentially lives. Scouts got killed, and scouts that never returned didn't give you information at all. They could only gather what they could see from a distance, and from a normal person's point of view. There's nothing quite like a birds-eye view when planning an attack on enemy forces. Knowing where they're coming from, when, how many men they have and so on is a great way to be able to strategize and attack the enemy at the opportune moments; real time scouting allows you to capitalize on opportunities when they arise, rather than needing to wait several days. Night vision allows them to see at night and be able to attack the enemy with ease, while things like radios allow for instant communication of amazing details, which Romans would lack. Their ability to fight not only extends to raw firepower, but sheer knowledge of what's going on, which in and of itself is generally speaking the key determining factor to who wins a war.
Despite the Superior Roman logistics, marines still possess a numbers of advantages. The typical MEU complement possesses 2,200 troops, nearly all of them trained how to fight like infantry, with a permanent ground force element of approximately 1,100 soldiers. They have 4 M1A1 main battle tanks, 30 Light Armored Vehicles, 63 Humvees, 6 155mm howitzers, 8 M252 81mm mortars, 6 attack helicopters, 6 attack aircraft, and 18 utility aircraft. In terms of logistics, they have 3 water purification systems, including the LMT 3000. They have 30 medium trucks, 4 tractors, 3 bulldozers, 4 general logistics vehicles and 7, 500 gallon water containers. The average marine consumes approximately 5.4 gallons of water day, for bathing, cleaning, and cooking, meaning the entire MEU consumes approximately 12,000 gallons per day. The Tactical Water Purification Systems can be carried on a Medium Tactical Vehicle Replacement truck, and can filter 1,200 to 1,500 US gallons (4,500 to 5,700 l) per hour. 8 hours of usage per day from a single unit is more than sufficient to produce the water needs required by the entire marine force. Although this consumes gasoline, the amount is small enough to be sustainable for many years. Marines are also experimenting with biofuels, that could be made from the natural environment. While this would only provide ideally up to 10% of what they need to maintain combat operations, it would be more than enough to maintain basic logistics. Until the generators, filters and so on broke, which are made to last for dozens of years, they could survive nearly indefinitely if they could receive water, food, and even fuel from the environment. With the other logistics vehicles, they could dig trenches, build walls, and fill sand bags. While this may seem insignificant, one sand bag can stop a bullet, and back in those days without things like aircraft and the like, they would be forced to pursue the marines on foot, and inevitably have to scale walls and trenches, which could be filled with spikes and other booby traps, in order to get to the marines. They could build these defenses rather quickly, and would provide a fairly significant advantage against mostly Roman infantry, allowing the marines to force the Roman forces to maintain a great distance between them, which would enhance the effectiveness of their ranged firearms even more.
Marines not only have the advantage in terms of raw firepower, but also in knowledge, science and strategic capabilities, as well. Little can replace the sheer advantage of having areal Reconnaissance or long ranged telescopes, and with Rome's all but completely inept anti-air abilities, they are completely defenseless against aircraft. In terms of firepower, they have machine guns, tanks, artillery, and even Areal bombardment, but all of this only lasts in the short term. Unless the marines can defeat and nearly annihilate the Romans in a number of key battles, they stand a poor chance of winning after they exhaust their limited combat resources.
The Roman War Machine
While the Roman war machine carried a great scope and range, expanding an empire that ranged between Spain and Turkey, incorporating Egypt, much of the middle east, Northern Africa, and most of Europe, including France, Italy and Britain. Their abundant natural resources, trade with the east and military power gave them incredible capabilities, and a nearly unparalleled reach. There's a saying "All Roads lead to rome", as Rome was known for building so many roads that, it was likely you would eventually get to Rome on one. From running water to transportation, Rome would possess capabilities and a population size not seen by Europe until the 1900's. The Roman Machine was able to maintain it's presence almost entirely through logistics, a strategy affectionately referred to as "99% economy, 1% military". They could not only march long distances and build siege engines, large walls and other structures, but they could maintain their operations nearly indefinitely due to their logistics and supply. For every soldier they lost they could replace them with ease, possessing both the money, general resources and the endurance for so few of their men to be lost. This gave the impression that the Roman legion was invincible and immutable, and would inevitably like a Glacier slowly inch across the world, until it had consumed everything. Every man was given their salt, and water, and elaborate water works, sewage systems and bath houses were built which dramatically increased the longevity of the Romans. Romans by and large felt special in the ancient world, as if they had things the rest of the world did not, and in general they were treated as such even by their most corrupt governments.
The Romans were insistent on every soldier reaching a certain minimum, in terms of equipment and training. Every single Roman soldier had to have a sword, an axe (fascis), a dagger, a spear, several javelines and a full suit of segmented plate armor. Compared to mideval armies, it was rare for the average soldier to have more than a gambeson or spear, let alone a sword. The axe, perhaps the most iconic weapon in the Roman eyes, severd as a pick, an axe and a hammer, and doubled as a tool for utility as much as a weapon of war. In the battle of Alesia, the Romans surrounded their enemies in just 3 weeks with 26 miles of defenses, and after facing attacks from the outside, surrounded themselves once again with defenses to stop the invaders. In just a matter of weeks they had built formidable defensive structures and siege weapons like the ballista, all by the skill of the soldiers, engineers, and of course the axe. The term "Fascist" and "Faggot" comes from this word, as the weapon was a representation for the Roman empire, and not only it's ability to fight, but for it's ability to build infrastructure, and build siege engines from nothing more than local wood and a handful of nails.
Romans could essentially travel anywhere, and from local wood and materials, build massive siege engines capable of taking entire cities. From ballista to trebuchets to catapults, the Romans could create massive war machines which could take out dozens of men at a time, nearly anywhere in the world. While the soldier himself was often little more than infantry, Roman ingenuity and skill allowed them to far exceed this limitation given enough time. Every Roman soldier was as much a worker to mine for metal wore, construct fortifications, build weapons and perform other such tasks as they were an outright warrior. The Roman way of war was to surround and squeeze their enemy, cutting them off from all outside resources, relentlessly attacking them, never letting go and beating down their enemy until they ran out of food and water, and other supplies and eventually died from a long war of attrition, or were forced to face the full might of the Romans in a single battle. Many conflicts were often settled without a single battle, as the Roman war machines presence alone could simply be enough to deter attackers from resisting. The Romans were unstoppable, incredible and immutable until, they eventually fell in the late 5th century. With the marines lack of logistics, against perhaps the most well funded and backed military of the ancient world, it would be a matter of pitting their best strategies against each other.