Coalition forces suffered less than 200 casualties in the 2003 invasion of Iraq, compared to the Iraqi casualties estimated in the tens of thousands. Similar casualty disparities exist in other modern conflicts. Was it a vast disparity in preparation between forces or is there something special about modern warfare that leads to casualty rates close to 100:1?
I'm going to answer the question using both modern and older examples.
Modern warfare is mostly a matter of a huge, extremely well-armed and equipped force using the best in modern technology, enormous amounts of training, and billions in budget, typically fighting guerilla fighters armed with whatever they could find or scrounge together.
In Afghanistan, as in Vietnam, Somalia, and other places, the people fighting against the coalition adopted guerilla tactics, because it is the only way to survive when wildly outmanned, outgunned, and out-trained by a vastly superior force.
An example in history of this is the battle of the Kokoda trail in WWII. A small contingent of Australian forces were dumped onto a tiny island in the South Pacific, and told to singlehandedly prevent the Japenese invading force from crossing the island and capturing Port Moresby, from where they were likely to invade Australia.
Rather than fighting them in a conventional manner, on open terrain, the Australians used unconventional guerilla tactics, for which they were publicly criticized by American General Douglas MacArthur and he fired their commanders for retreating.
Despite that, the tactics worked, and Japanese suffered massive casualties, and eventually left.
Another good example is the Battle of Rorke's Drift - ~150 British soldiers armed with rifles against 3-4,000 Zulu warriors armed with spears, and the British won (also the most Victoria Crosses handed out for a single campaign).
So imagine you're an Iraqi soldier facing up against an M1 Abrams, with artillery pieces so accurate they can hit a target 10cm across, with GPS locators in the fuses, weaponry that can punch through a conventional tank like a hot knife through butter. Miniguns that fire tens of thousands of rounds a minute, and soldiers wearing not only advanced armour, but infrared strobes, so their aircraft can distinguish them clearly at night, all while continuing to fire a few million rounds in your direction while being able to identify their own troops as clear as day. Still wonder why the casualty numbers lean so heavily in favour of the best armed?
From the rules:
No current events
To discourage off-topic discussions of current events, questions, answers and all other comments must be confined to events that happened 20 years ago or more, inclusively (e.g. 1995 and older).
This is probably the wrong subreddit to ask this question.
I think I can answer this question in a way that fits the rules of the sub by substituting the 1991 Gulf War for the 2003 invasion of Iraq. It is estimated in a report by the US Air Force that between 20-25,000 Iraqi soldiers were killed between the air campaign and the subsequent ground invasion, while Coalition combat deaths were 146.
The biggest reasons for this massive disparity are technology and the strategy that Saddam pursued.
First, the massive superiority in technology that the coalition enjoyed. In the books The Insurgents and The Generals, Fred Kaplan and Thomas Ricks, respectively, argue that following the quagmire of irregular war that characterized the conflict in Vietnam from 1965-1972, the military set out to fundamentally reorganize itself. The military refocused on fighting large-scale conventional wars in preparation for a potential World War III scenario to be carried out against the Soviet Union on the plains of Eastern Europe. Aided by the heavy defense spending of the Reagan administration, the military pumped huge amounts of resources into developing new weapons systems to fight this kind of war: The F-16 fighter, the M-1 Abrams Tank, and the Bradley Fighting Vehicle, for some examples. These weapons would prove to be incredibly effective in a conventional struggle like the Gulf War where the Iraqis chose to try and fight set-piece battles with inferior, outdated Soviet-era equipment.
This brings us to the second reason: Saddam foolishly thought that he could face the American-led coalition in a conventional war of set-piece battles. Iraq had just emerged from the eight year long struggle against Iran (paying for the massive cost of that war was one of the primary reasons that Saddam invaded oil-rich Kuwait in the first place), a conflict that was marked by bloody stalemate and frightful casualty rates that echoed the trench warfare of World War I. Saddam believed he could replicate that kind of conflict with the Americans, making the cost of victory to high for the American public to stomach.
This plan was foiled, however, by the massive advantages in airpower and technology that the coalition possessed as well as the decision to carry out the great "left-hook" where US troops invaded across the supposedly unnavigable Saudi-Iraqi border dessert rather than through the heavily fortified approaches to Kuwait itself, an attack enabled by the use of recently developed GPS systems.
A telling engagement during this attack was the battle of 73 Easting, in which the American 2nd Armoured Combat Regiment destroyed 17 Iraqi tanks and over 70 other vehicles at the cost of one Bradley disabled and one soldier killed. Then-Lieutenant John Nagle, who would go on to become an expert in the field of counterinsurgency, recounts in The Insurgents how it was 73 Easting that made him realize how dominant the United States had become in conventional wars, and that it would be basically suicidal for any potential opponent to attempt to fight the United States conventionally in the future.
This leads us into a discussion of the counterinsurgency campaigns that the US has become involved in in the past decade and half in both Iraq and Afghanistan, but that discussion would violate the sub's 20 year rule.
So, to summarize, it is not so much the case that modern war inherently have massive casualty disparities, but that that is the result of a matchup that pits a very strong and technologically advanced opponent against a much weaker one in a style of conflict that also plays to the advantages of the stronger (something that characterizes both the Gulf War and the 2003 Iraqi Invasion, but not the following counterinsurgency struggle that developed following 2003's invasion).
Disproportionate casualty ratio is not limited to modern wars, neither do all modern wars produce a disproportionate casualty ratio. The casualty ratio of a war is very dependent on the balance of military power between the opponents, but also on their respective war-readiness and the actual execution of their military operations.
There are several modern wars (within the last 50 years) that didn't produce disproportionate casualty ratios. In the Falklands War, Britain incurred around 255 KIA (killed in action) while Argentina incurred around 655 KIA [BBC]. In the Sino-Vietnamese War, the ratio wasn't severely skewed either; but I won't provide figures because the whole thing has gotten so badly muddled in politics. The Yom Kippur War produced a casualty ratio – i.e. dead and wounded for Israel versus the Arab nations – that is greater than 1:7, even if going with the most loose estimates. I personally think the ratio is closer to 1:3, but the fine details of that war is not my forte. The estimates for the Iran-Iraq war is all over the place, but very few cite a ratio more disproportionate than 3:1 for killed, missing and captured.
Moreover, there are non-modern wars (beyond the last 50 years) that produced very disproportionate casualty ratios. We can start with the Battle of Marathon (490 BCE), where 213 Greeks and 6,400 Persian died against eachother, according to Herodotus. There is also the Battle of Cannae (216 BCE) and the Battle of Carrhae (53 BCE), both of which produced a terribly disproportionate slaughter of the Roman legions. You should check them out, they're very interesting battles. In more recent times, we have the wider Battle of Prokhorovka (July 1943) for which German archival reports show that they lost less than 17 tanks and assault guns (these are only irreparably destroyed or abandoned vehicles), whereas recently published Soviet archival documents shows that the Soviets lost 334 tanks (these are only irrecoverable losses) self-propelled guns. There are many, many more examples of battles and wars that produced disproportionate casualty ratios.
If you have any specific questions on how balance of military power, war-readiness and the actual execution of military operations can affect casualty ratios, feel free to ask. My prime forte is in WWII Eastern Front and Ancient Near East history, so I might be able to use examples from these areas to help.