This is unrealistic view.
US military is well-funded and well-equipped but parts of it are committed to it for different ends around the world. Just check US military commands.
In case of war with a country, US does not redirect its entire force composition to the theater; not even close. Some of the security commitments are too important to be left unattended in American calculus.
The opposing country does not have similar constraints. It can fight as a whole. This was the case with North Korea, Vietnam and Iraq. This is true even in the case of Afghanistan.
When you are on the offensive in hostile territory, conventional wisdom is to outnumber your enemy (minimum disparity = 2:1; ideal disparity = 7:1). But Americans do not follow this rule and try to compensate with technology. They can win battles with this approach
but occupation of hostile lands and counter-insurgency are more demanding roles in comparison.
To give you some perspective, Indian military controls Jammu & Kashmir with "sheer numbers." It does not hold a candle to US military in conventional warfighting (and COIN), however.
So let us have a recap of some conflicts.
Korean War (1951 - 1953)
American military footprint was small in Korean peninsula
before North Korea invaded South Korea and virtually annexed it in 1950. General McArthur opted for a tactical masterstroke (Operation Chromite) to catch North Korean military offguard and rout it from South Korea. When US-led forces were inside North Korea, China decided to help North Korea and pushed them back all the way to Pusan. US-led forces had to regroup and recapture South Korea. Korean War proved to be one of the most difficult to fight in any sense. Every stakeholder suffered heavy casaulties in the process.
BACKGROUND North Korea and South Korea were split across the 38th parallel by USSR and USA after World War 2. USSR helped transform North Korea into an organized communist state with well-equipped armed forces in the (1945 - 1950) period. USA did not pay much attention to security needs...
defence.pk
Vietnam War (1955 - 1973)
France was in control of Vietnam
before World War 2. Japan managed to created Vietnamese insurgency against France and the First Indochina War broke out in 1946 which concluded with defeat of the French in the
Battle of Dien Bien Phu in 1954. Vietcong could commit a massive force of 80,000 men for this battle, and prevailed.
Vietnamese demonstrated considerable potential in guerilla warfare and even in conventional military operations. USSR and China provided much-needed arms to them for obvious reasons.
Loss of Vietnam alarmed many in the West and USA decided to intervene and preserve friendly South Vietnam. The Second Indochina War broke out in 1955 and concluded with the fall of South Vietnam in 1975.
There was another problem, however. Vietnamese terrain
favors guerilla warfare and provides very limited space for mechanized military thrusts.
Vietnamese terrain made it impractical for USA to fight effectively on the ground like in World War 2. Military technologies of the time were not much better than in World War 2 either.
Although, American troops could fight and win battles in Vietnam, political situation of Vietnam did not shift over time. South Vietnamese were utterly lacking in capacity and appeal to reunite Vietnam as well. Ronald Reagan noticed the obvious and pulled the plug in 1973.
Persian Gulf War (1991)
Iraq was at war with Iran in the 1980s on religio-political grounds. Iran was better equipped and motivated than Iraq and turned the tide of war in 1982. Some countries decided to help Iraq and provided much-needed arms and technical knowhow to it. Iraq was able to rebuild its army and defeat Iranian forces in 1988 (Operation Ramadan Mubarak; Operation Forty Stars; Tawakalna ala Allah missions). In the same year, USN defanged Iranian Navy to bring an end to the ongoing Tanker War (Operation Praying Mantis).
Iraq now had a battle-hardened and well-equipped military force at its disposal. Tensions with Kuwait spiked over economic concerns, however. Iraq literally walked over Kuwait in under a day in 1990. GCC bloc was understandably spooked and requested American intervention.
KSA provided much-needed space to US-led forces for military buildup (and maneuvering possibilities) in case of war with Iraq. Battlefield tactics were given due importance in fact.
Vietnam War had taught many valuable lessons to USA in regards to how to develop equipment which can be more effective in difficult landscapes. A new generation of military equipment including sophisticated cruise missiles, jet fighters, stealthy strike platforms, surveillance systems, SAM systems, and heavily armored main battle tanks were conceptualized and produced in good numbers. Advances in computing technologies paved way for these innovations and development of network-centric military operations. Some of the technologies were relatively unknown to many around the world
until the war broke out in 1991.
Iraq committed estimated million troops for anticipated war with US-led forces. These troops were heavily concentrated in Kuwait and within Iraq near Kuwait. The fundamental idea was to make counter-invasion of Kuwait very costly to the adversary [1].
General Schwarzkopf was not taking any chances, however. US-led forces composed a million troops as well (parity with Iraq). Breakthrough in military conflict was now contingent upon surprise factor in both tactics and military technologies.
Iraq was led to believe that Normandy Landings will be replicated in Kuwait. This impression helped inform Iraqi military positions accordingly (see [1] above). In reality, thrust would come from KSA and Iraqi armed forces will be encircled and cut-off from within Iraq and forced to surrender consequently.
Back in World War 2, Supreme Commander of allied forces Eisenhower had a similar plan. Germany was led to believe that US-led forces will invade France from Africa. This impression helped inform German military positions across France accordingly. In reality, thrust would come from UK (Normandy Landings). Due to this development, some of the German military divisions could not reach Normandy in time and all were routed one-by-one.
Point is that Operation Desert Storm was brilliantly executed with the right composition of manpower, technologies and tactics. The outcome surprised much of the world including USSR and China.
Some of the Americans were surprised themselves because they were taking Soviet hype of Iraq at face value.
Pakistani COAS of the time Aslam Beg was also of the view that US-led forces will suffer heavy casualties in war with Iraq and this saga would be a repeat of Vietnam. He was proven wrong.
The bottom line is that none of the conflicts you mentioned were easy to fight, and each conflict presented its own set of challenges to Americans in the battlefield. In all fairness, these were among the most challenging conflicts of modern times.
Sheer downplay and underestimation of the warfighting capacity of countries other than USSR and China is
not helpful.
Please bear in mind the fact that a country which is invaded or finds itself to be in defensive role, is not necessarily alone. External assistance can materialize for some in these circumstances.
I never asserted that technology alone decide wars. Objectives should be realistic and attainable. Tactics should be given due importance as well.
This discussion is about a war which will be fought in a piece of land that is not a part of either Russia or China. In this relatively neutral venue, there is no guarantee which camp will prevail.
Taiwan
can be successfully defended in view of its geography, technological supremacy and warfighting experience of US-led forces
in theory. Whether this level of support will materialize or not is unclear - a matter of political will to say the least.
Taiwanese insurgency will have to show potential much like Vietnamese insurgency nevertheless. Counting entirely on Japan and USA is shortsighted. Taiwan is located very close to China - this arrangement have a psychological impact of its own.
But to assume that Russia or China can fight and win a war anywhere in the world is also shortsighted. Modern war can be unpredictable for any side.