The Japanese were truly shit soldiers for the most part. The Japanese Army really only excelled at killing unarmed civilians on the Asian mainland. Japan lost 14 soldiers killed for every 1 American/Allied soldier they managed to kill, a terribly pathetic ratio. No nation worth a damn would ever field a force so incapable that it cost them 14 of their own soldiers for every enemy soldier killed, particularly in light of the respective populations of Japan and the USA. If Japan had out-numbered the USA 40 to 1 in population, then trading 14 to 1 in losses might have been a reasonable strategy of attrition. In 1941 the USA had about twice the population of Japan, thus Japan's situation of trading 14 of their soldiers for every American soldier was not sound.
Now the Japanese did manage to kill about 30,000,000 Chinese civilians who were basically unarmed peasants. Most Americans cannot wrap their minds around the Chinese or Soviet losses in WW2, about 30,000,000 dead Chinese and 27,000,000 dead Soviets/Russians [about 9,000,000 of the Soviet losses were military, the rest civilian].
The USA entered WW1 in 1917 and fought a depleted worn out Germany for a few months of 1917 and then 1918.
The USA entered WW2 in Dec 1941, the last major power to enter the war, and then fought depleted Axis forces in North Africa in 1942-1943 in a theater the Axis considered a distant second to the Eastern Front, then fought second-rate and third-rate Italians in Sicily and Italy in 1943, then fought a handful of depleted German divisions in Italy from 1943 to the end of the war [the Germans dragged out the defensive campaign for 2 years], and then fought depleted second rate German divisions, Ostlegion, Static Divisions, and a few SS and serious Heer divisions in the West from 1944-1945.
The Americans went through WW1 and WW2 entering late and fighting depleted worn down powers who were definitely not in their prime.
The Americans should have handily won the Korean War but barely managed to achieve a stalemate. Think about it, with largely the same military from 1945, a lot of fresh and recent combat experience, institutional experience, a war industry that was still intact, no damage to the homeland; the USA should have been able to crush North Korea and the Chinese interlopers like insects. China wasn't even a near peer let alone a peer, it was a depleted worn out quasi feudal agrarian nation which had just come out of a civil war and before that it had been at war with Japan for almost 15 years. Mao's China should have been thoroughly trounced in Korea.
Beating Japan in the Second World War wasn't a huge accomplishment because frankly Japan sucked. Their economy was 1/15th the size of the USA and their basic soldiers were not particularly capable. Japan was delusional to think that attacking the USA would have positive successful results.
Could the USA have defeated the European Axis in their prime without the Soviet Union being involved? Highly unlikely. Only if the Western Allies maintained a continental blockade and starved the European Axis of crucial resources and food, similar to what the British blockade in WW1 was aimed at, although it would have been more difficult in the 1940s with Germany occupying most of Western Europe and the Soviet Union trading with Germany.
The USA does not have a particularly impressive track record for winning wars against strong competent powers. A lot of Americans insist that the USA could win a war against Russia, China, Iran, and North Korea, at the same time, while it isn't clear to me that the USA could defeat any one of those nations alone, let alone a combination of them or all of them simultaneously. Unfortunately the USA is probably going to have to lose a war and have a Versailles style peace inflicted upon it, before Americans realize, acknowledge, and admit just how weak the USA is and how far the USA has fallen.
Our unrealistic national self-image makes it far more likely we will blunder our way into a major global war, convinced of our own invincibility, setting ourselves up for ultimately being humbled, humiliated, and possibly forcibly disarmed of our navy and strategic assets or even occupied, in the aftermath of the defeat.
The United States is one of the least capable major powers when it comes to achieving successful military operations and prosecuting major wars against major powers, yet we view ourselves as being the most capable and even invincible. Our image of our nation doesn't match up with the fairly unimpressive military history of the United States. This disconnect with reality is likely to lead us to make major policy plunders in regards to our actions on the world stage.
Imagine a local tough guy who is convinced he is tough because he has made a name for himself beating up old men who aren't train and cannot effectively fight back. When he finally encounters somebody who is an actual fighter, he is going to get it.
The US has accumulated 20-25 years of experience fighting what amounts to armed civilians operating as insurgents in Iraq, Afghanistan, Yemen, Somalia, etc. The USA has no recent experience with ship-to-ship combat, air-to-air combat, or ground combat against conventional combined arms forces. I believe the last time the USAF had aerial combat experience was in Vietnam. The last time the USA engaged in ship combat was in the 1980s during the Iran Iraq War [very limited], somewhat in Vietnam [very limited], mainly in WW2 [quite a long time ago]. The last time the USA fought against any semblance of combined arms forces on the ground would perhaps be Desert Storm, prior to that, some aspects of Vietnam, really you have to go back to Korea or WW2 to find real serious examples.
Since the 1950s the US Navy has enjoyed almost complete supremacy anywhere they have operated. The US Air Force has enjoyed aerial superiority and usually total aerial supremacy, in almost every theater or war. I believe the last time Americans were on the receiving end of shelling by anything larger than a mortar, it would be howitzers in Vietnam. American volunteers in Ukraine were not doing well being on the receiving end of overwhelming Russian air and artillery assets.
Fighting is a lot easier when you have complete control of the air, you have an AC-130 on stand-by, you can call in an A-10, the enemy has no night vision, and the enemy is a collection of armed civilian farmers and goat herders who have nothing larger than 82mm or the occasional 120mm mortar, but more likely RPG-7s and PKMs. When you're suddenly faced with a Russian Guards Tank Army of 60,000 men in a combined arms formation where they have overwhelming artillery, air power available, and the average soldier has at least 6-18 months experience in Syria and many had experience in Georgia or the Second Chechen War, and many had experience in Donbas from 2014 forward, it is a different situation.
For what the United States taxpayers spend on the military, the realized return is not impressive. It is a pretty bad investment in financial/economic terms.
Fortunately for the USA, almost all wars [except those like the Six Day War that are over too quick for war economy/industry to be relevant] are generally decided by antecedent economic, demographic, and logistical factors. The USA tends to "win" wars through economics, demographics, and logistics, which is a large part of how Republican Rome won most of its major wars.
As long as you're okay with taking a lot of casualties and spending a lot of money, there is nothing "wrong" with the American approach to "winning" a war. Hannibal crushed Rome in 4 separate battles, with the fourth being the Battle of Cannae whereby his forces killed about 50,000 Romans and captured at least 15,000 which effectively destroyed Rome's field army. Rather than surrender the Romans simply raised more legions and fielded another army. Most people cannot fathom 50,000 young men being killed in one day, in one battle, those sort of losses in that short of a time-span are mind-boggling to almost everybody who tries to contemplate it.
en.wikipedia.org
Keep in mind Rome had lost about 20% of its adult male population as the result of 20 months of combat [4 major battles] against Hannibal. But Rome resolved to raise another army and fight on. Would the USA be okay with losing 20% of its adult male population? I don't know. Should the USA ever be okay with such a proposition? Probably not, not unless the situation were utterly dire, critical, and the survival of the very nation was at stake, then it might be worth considering acceptance of such losses.