Why was the Japanese Army's fatalities inflicted:suffered ratio so low in the Pacific theatre?

As NSNoob has stated, the Japanese did do very individually, and in small groups when barely adequately supplied. Their land forces normally did so well that their reputation alone took Okinawa and the Philippines at the start of the war. In fact, the fighting soldier is the last link in a long supply chain (logistics), involving the entire industrial manufacturing chain. The Japanese problem began at the top of the chain, not at the bottom. They ran out of ships, pilots, and supplies. The Japanese took all those islands and China in order to secure resources for the war effort to hold said resources. They began the war with the strongest (numbers and effectiveness) Navy in the world at the time, and built only a few ships while the war was in progress. The US began the war with perhaps the weakest in numbers, effectiveness and training (take the fact that Pearl Harbor was a success at all as an example) and ended the war with a 1000 ship navy. The Japanese saw the writing on the wall long before the Allies did and built 25 aircraft carriers, but they didn't replace them as they sank (no resources or manpower) and couldn't/didn't train pilots to fly them. So the individual soldier was essentially by 1943 swinging gently, gently in the wind. The stagnation of the Japanese war machine was the result of the impairment of the entire resource manufacturing chain as NSNoob alludes. There was no absolute guarantee that the Japanese were going to come down on the Axis side until the UK saw their increasing naval strength and moved to block their access to resources. This put them firmly in the Axis camp. The UK had a several hundred year policy of maintaining the strongest single navy in the world, and moved to block any other country that attempted to contest this. Supply chain movement by weight tells the story, the US could put 10lbs per solider per day on the ground and the Japanese averaged around .1, the Germans 2lbs (before Casino). The IJF tactical manual estimate 4.2 pounds per day.

I'm not an expert on this field, but just wanted to put out this thought, based on several historical studies.

The thought is, didn't the disparity really start to show up after the strategic naval losses? By that point, the defensive fights that the Japanese were fighting were almost exclusively scenarios where they were beseiged (aka, stuck on a island with no viable means to evacuate), critically low on food and water, and so low on ammunition that their combat response was affected.

The IJA forces stuck on the islands were basically left to die a slow death and given the coup-de-grace by an overwhelming amphibious landing force supported by naval forces as well as by air. I got the sense that many of the battles might have seen other national forces just surrender, except for Japan. IJA soldiers exhibited a fanaticism that trumped low morale time after time. I mean, look at the Italian forces who also had good soldiers but suffered from poor food, low ammunition, and essentially saw their forces suffer a catastrophic morale and organizational collapse due to these failures. It's pretty amazing that the Japanese soldiers refused to surrender and insisted on fighting even in those situations where they pretty much faced suicidal odds. And I think it's this fanaticism that led to the ratio that you're talking about. Without the fanaticism, the ratio would have been much, much lower, and the prisoner-of-war intake would have been significantly greater for the US forces.

I'm just thinking of the battles of Saipan, Okinawa, and Iwo Jima - but there were many other cases where the Japanese defenders, when faced with imminent, hopeless odds, stubbornly insisted on a bitter contest of arms, despite being unsupplied and low on ammunition.

Anyways, just my humble $0.02 worth.

I wanted to add a few thoughts and tie it all together.

Short Answer: After the first few months of the Pacific war the industrial differential between the two countries increasingly lead to a colossal mismatch in logistics, arms, supplies, and troops. The beginning and early conduct of the war, justified almost any barbarity in matching the enemies fanatical nationalism. In the latter period of the war the United States demonstrated it had more than a bit of fanatical nationalism of its own. No Quarter given on either side of that theatre of WWII.

Longer Answer: First off, Japan did have fine fanatical soldiers well trained and motivated; as has been previously stated. That had nothing to do with the lopsided casualty rates. If anything their fine military would have lessened the lopsided casualty rates Japan suffered after the first six months of WWII. The lopsided casualty rates came from two factors. (1) Japan picked a fight was an industrial super power which they never had a chance of winning once the war became one of attrition. (2) How Japan picked that war, and their conduct in the execution of that war when they had the upper hand was by most American accounts cruel, honor-less and pitiless and that's exactly the emotions they faced in return as millions of Americans volunteered to go to the Pacific and sort out this enemy.

(1) Japan wasn't an industrialized country. Japan at the onset of WWII could muster at best about 10% of the United States industrial output. They were a feudalistic agrarian society with an oversized miltary. The United States was already the leading industrial economy in the world, with a rather undersized military in 1941. A war of attrition, which is what WWII turned into, was not ever going to favor a Japanese victory. Japan's strategy was to hit hard and hope for a negotiated peace. When that failed so did any chance for a Japanese victory. Regardless of the quality of their soldiers they were outclassed by US production across the board. Take the battle of Iwo Jima for example..

source

183,000 U.S Army and Marine Corps troops, carried in 430 ships and craft, and over 747,000 measurement tons of cargo

That's more than 4 tons of logistical support for every sailor, soldier, and marine who participated in the Battle of Iwo Jima. Compare that with the logistical support the 20,000 Japanese defenders. Japan planned for logistics support for their soldiers measured in lbs per soldiers, not tons.

Another Example... How many Capital Ships did the United States produce between Dec 1941 and 1945? Literally thousands. How many did Japan produce. None.. They basically fought the war with what they had. They lost 4 aircraft carriers at midway, and never produced a single fleet carrier which production started after Dec 7th...

Source

At its peak, the U.S. Navy was operating 6,768 ships on V-J Day in August 1945, including 28 aircraft carriers, 23 battleships, 71 escort carriers, 72 cruisers, over 232 submarines, 377 destroyers, and thousands of amphibious, supply and auxiliary ships.

Source
Ship Class------------------US (Japan)
AirCraft Carriers-----------124 (18)
Battle Ships----------------8 (2)
Cruisers--------------------48 (9)
Destroyers------------------349 (63)

The logistical mismatch gave the United States the means to utterly destroy the Japanese wherever they found them.

(2) As for the motivation, it cannot be emphasized too heavily how angry both the sneak attack on Pearl Harbor made the United States, and the subsequent poor treatment prisoners of war endured at the hands of the Japanese. After Pearl Harbor the recruits who went to the Pacific believed they were in a fight with godless pitiless barbarians, and they were there to match that pitiless behavior. The Marines at Guadalcanal fought so fiercely that the Japanese believed the United States had paroled murderers and criminals to fight against them. The Marines loved that description so much it became part of the Corps Lore.

Here is a good article which details the wide spread mutilation of Japanese soldiers, living and dead which occurred during WWII.

Here is an image from Life Magazine published in 1944.

Photo published in the May 22, 1944 issue of LIFE magazine, with the following caption: “When he said goodby two years ago to Natalie Nickerson, 20, a war worker of Phoenix, Arizona, a big, handsome Navy lieutenant promised her a Jap. Last week, Natalie received a human skull, autographed by her lieutenant and 13 friends and inscribed: ‘This is a good Jap-a dead one picked up on the New Guinea beach.’ Natalie, surprised at the gift, named it Tojo. The armed forces disapprove strongly of this sort of thing.”

Clockwise from top left: U.S. soldier with the Japanese skull adopted as the “mascot” of Navy Motor Torpedo Boat 341 circa April 1944, U.S. soldiers boiling a Japanese skull for preservation purposes circa 1944, a Japanese soldier’s severed head hangs from a tree in Burma circa 1945, a skull adorns a sign at Peleliu in October 1944.

Here is a newspaper article from Nevada Daily Mail (see second column under "Sign of Nervousness") which talks about Roosevelt receiving a letter opener made out of the forearm bone of a Japanese soldier and he responds that "that's the kind of gift I like receiving".

So wide spread was the mutilation of Japanese soldiers that twice the Army Chief of staff had to order directives against the practice.

Throughout the United States’ campaign in the Pacific, American soldiers indeed mutilated Japanese corpses and took trophies — not just skulls, but also teeth, ears, noses, even arms — so often that the Commander-in-Chief of the Pacific Fleet himself had to issue an official directive against it in September 1942. And when that didn’t take, the Joint Chiefs of Staff were forced to issue the same order again in January 1944.

Why were the Japanese military fatality rates so high during WWII against the United States. Because from the American perspective of the period, those are the kinds of seeds the Japanese sowed and that's the resulting harvest the Japanese reaped.

My uncle was a Marine in these battles. His generation spoke very little about the war. He was in the Pacific. One day we were discussing wars. He turned to me and said:

"You know we did not take prisoners...".

There were many reasons for this:

1. There was no place to put or hold prisoners.
2. You had to be constantly on alert with Japanese prisoners since they were adept at unarmed combat and would prefer to die while killing an enemy
3. It was well known that the Japanese had beheaded soldiers who surrendered. They believed a soldier that surrendered had no honor

Given all those considerations, it was simpler and easier "not to take prisoners..."

Japan had a disadvantage in heavy equipment, especially artillery and ships artillery. Many Japanese soldiers were killed in heavy bombardments. After the early battles (e.g. Guadalcanal), Japan seldom bombarded or bombed American soldiers. Artillery fire is the deadliest in combat, more so than "small arms" fire because it is heavier; deadlier than say, bombs, because it's closer to the ground.

In some ways, the Japanese casualty rate was not that much higher than that of the Americans. If you take casualties as "killed and wounded," the Japanese to American casualty rate was less than 2 to 1, and sometimes as low as 1 to 1. What is true is that the Japanese ratio of killed was much higher. The reason was that essentially all Japanese casualties were "killed" while most American casualties were wounded.

Okinawa is a case in point. Wikipedia lists American casualties (to the nearest thousand) as 20,000 dead, 55,000 wounded, 26,000 psychiatric, that is 75,000 physical casualties, 101,000 in all. Japanese casualties were estimated at 77,000-110,000 dead, 7,000 known be captured, approximating the American toll.

One disadvantage the Japanese had was that they were trapped on Pacific islands. American "wounded" were usually evacuated and nursed back to health; Japanese wounded were wounded repeatedly until dead, or they killed themselves. Few surrendered (the 7,000 at Okinawa was "high" in this regard).

I want to supplement NSNoob's answer with some more information on Japanese small arms. They lacked the firepower which the Americans could bring down, firepower which is very important in obscured and close range jungle fighting.

Compared to the Chinese, their primary land opponent, the Japanese army fared fine. This is something very important to remember, the bulk of the Japanese army was fighting in China. The Chinese army was poorly supplied, poorly trained, and the Japanese arms and tactics did fine against them. Compared to the Americans, their primary opponent in the Pacific, Japanese equipment and tactics were obsolete.

For example, the standard Japanese rifle was the Type 38 and Type 99 "Arisaka". This was a very long bolt action rifle from the turn of the century. It was a good bolt action rifle, but it was still a bolt action rifle. In contrast, every US solider had the semi-automatic M1 Garand. In terms of sheer firepower, a US army unit of equivalent size could pour out more aimed shots than a Japanese army unit.

Source. That's a Japanese soldier with a Type 38, just to give you an idea of how long and unwieldy this rifle was.

On a similar vein, Japanese troops lacked a good submachine gun for mobile, short range firepower. The US had the Thompson, the M3 "Grease Gun", and the BAR.

Japanese did have excellent machine guns. The Type 96 and Type 99 Nambu light machine guns (video on that from Forgotten Weapons) and the Type 92 heavy machine gun (and a video on that).

Source. That's a Type 99 light machine gun, and the bayonet is not a joke. It was a standard feature and says a lot about Japanese army mentality that they would put a bayonet on a crew served weapon.

The Japanese were entirely capable of producing better small arms, but nothing in the millions like they did the Arisaka, and little got to their island garrisons. They did have a submachine gun, the Type 100, but only produced about 25,000. They even copied the Garand (video about that copy) and Pedersen (video) semi-automatic rifles. By the time they realized they needed a better rifle, it was too late. Their production and transport system was a shambles.

This was due to a combination of limited production capacity, the pressures of keeping their army in China supplied, the supply problems caused by the Allied interdiction campaign, the lack of a private arms industry to supplement and innovate, and a general feeling that soldiers and spirit won battles.