It's designed for a mass engagement against another superpower. Most of their arsenal is dedicated to countering the best weapons of a country like Russia or China, which they've lagged behind in due to the parasitic defense contractors. You can't achieve a counterinsurgency campaign with bombs though. Killing one civilian radicalises twenty. As an occupation force the US doesn't have enough public goodwill to fill the role of UN peacekeepers and they don't invest enough in reconstruction/provision efforts to build that goodwill.

capitalism. most of the money goes to execs. investing into shitty expensive toys to funnel more money to execs etc.

When you have to pay almost $5000 for a replacement antenna that costs the MIC $5 to make, you're going to get F-15s held together with duct tape.

Because the money isn't going into training or reliable weapons tech but flashy wunderwaffe and the maintenance of relics that should've been retired 40 years ago.

Indigenous terrain advantage of the invaded nation.

Many good answers in the replies but one I haven't seen is that military departments in the US lose their budget if they don't use it which creates a huge incentive to be extremely wasteful. Especially in aerospace.

It’s not so much that they’re underperforming, but that Iran, Russia China and North Korea have been following US operations and have been working on counter measures.

All of these countries understand the importance of electronic warfare.

Iran has spent 38 years establishing supply lines and keeping up with tech to defend against an invasion.

The US does not have incentive to defend against invasion, so they’ve been lax in R&D

Remember when they said the Russian military is just bloated waste going directly to corrupt generals and politicians rather than into equipment? Yeah, it was projection again.

Its not an efficient organization, its full of private monopolies leeching/laundering money in every step of the way.

Skill issue. Don't blame the tools!

My answer is because they aren't fighting for a just cause. So they lack the grit to grind it out for years or decades. It's built for and on quick wins.

Underperform? It is operating just as Wall St demands. It's a goddamned profit making juggernaut. They are now talking about a $1.5T budget. (sp)

Indi has a couple articles on the subject from a couple years ago:

https://indi.ca/nothing-to-see-here-just-the-wheels-falling-off-empire/

https://indi.ca/how-americas-military-has-fallen-apart/

The US military can still do damage even in its state of decline, but as Iran is showing, its two main strengths are air defense and aerial bombing (air superiority more generally), and if you have enough missiles to overwhelm its air defenses and destroy its early warning systems, and you have enough air defenses to stop it from wreaking havoc just anywhere, the cracks start showing fast. Its navy is clearly struggling and in the context of Iran, ground forces would struggle to gain any traction due to the terrain.

This isn't a full explanation since I'm just dictating real quick, but I'm assuming this will build on what other commenters have added. The U.S. state is privatized and fundamentally opposed to capital controls. In order to have an effective industrial warfare strategy, it would need to actually apply pressure to investors in the military industrial complex, other than saying, have at it make as much money as you can. The U.S. doesn't suddenly sprout factories when it gets additional funding in the MIC. It makes the weapons more expensive.

For over 30 years the US has only fought let's say sub-peer militaries. Vietnam was a factor that prompted them to innovate briefly - we often think of Vietnamese soldiers as 'farmers' but no, it was a comprehensive, exhaustive, and almost peer military. They shot down planes all the time and led a two-prong strategy of guerilla and conventional (with the army in the north alongside what is referred to as the Viet Cong which was a mass front, i.e. federating all vietnamese into it regardless of affiliation, though under the command of the communist party). They were geniuses, you don't win on grit alone.

The M16 and M4 were introduced in Vietnam for example, and of course the extensive use of napalm and agent orange and new helicopters made for the theater. It might also very well have been the first US "forever war".

But I would position Vietnam as a turning point because after that, the US never really fought the same wars again. There was the Iran invasion by Saddam that was financed and armed by the US, then after that the hostage crisis, the grenada invasion, invasion of Panama, first Gulf War for Kuwait, destruction of Yugoslavia, Afghanistan, second invasion of Iraq, Libya in 2011.. frankly the list goes on, I'll just put it in a spoiler. It's huge, and frankly I'm not even sure it's exhaustive.

But we can see that after Vietnam, maybe even after World Ward 2 (!), they just stopped fighting against peer militaries.

So that's the first factor. As they stopped fighting against peer militaries that could take their planes down easily, that could bomb their bases, that could force their ships away from the theater, even someone who could bomb US territory like the Soviet Union could, they necessarily focused on different strategies. You need to find out guerilla troops for example, right? Their entire point is they try to hide, sometimes even in civilian populations (aside: in terms of para-military you also have the guerilla, and the resistance. The resistant has a day job, he looks and acts like a normal civilian, but he runs operations for the resistance). That's why you see stuff like the Anduril helmet that 'sees' through walls (https://x.com/0xmitsurii/status/2039201367294955707). The way it works if you watch the video is soldiers and drones deployed in an area communicate together with this helmet and assets are marked for everyone as long as one of the devices can see it. So it doesn't do X-ray vision, it's more like a minimap with a tag mechanic. But regardless, that's great for anti-guerilla operations! Not so much useful when your bases are being bombed by hypersonic missiles, or when you can bomb the data center that coordinates and computes all this tech. Too little too late as always.

Regardless, you still have a defense industry. The defense industry in the US was created for world war 2, because they needed to arm themselves quickly, so private contractors emerged and were strengthened. Eiseinhower was apparently the one who coined the term military industrial complex in 1961. These 'contractors', i.e. private businesses, still need to exist after the war though, because otherwise you have no real army capacity anymore. Especially as the US emerged as the imperial hegemon after the war over the world, they needed to expand this military to enforce their order, so that means more spending.

This leads to bloated spending. Yes the companies want to make money, but they also need to stay alive. You see this in construction in some places (I'm sure the US does it too), where the government hires them to fix stuff that doesn't need fixing because we don't need them that much, but they still want them to exist. Of course, this makes them a lot of money.

A company like Anduril doesn't care about saving soldier lives. Not directly at least. They care about selling their product over the competition and making billions off of it.

So spending bloats up, and the state stops fighting peer enemies and switches strategy to handling insurgency, guerilla tactics, etc. They take air superiority as a given, they take the defense of their bases as a given (they were never in any real danger in Iraq and Afghanistan), they take their 'freedom' of navigation as a given, being able to deploy their ships wherever they want, and build an entire system based on that. Aircraft carriers are now sitting ducks, though still heavily protected by an escort. A hypersonic would make short work of it if it can penetrate the envelope of defense at hyper speeds. Likewise the US doesn't need hypersonics because they don't normally fight adversaries that can intercept any missile - interception is really hard, I'll give them that, and the US excels there compared to other militaries. We can point to Russia as a peer military but even they don't have amazing interception capabilities.

But then we get into the operational and logistical question.

Have you seen the pictures of rusty US ships?

This is more complicated for me to answer, I'm not too deep into the operational side. But operationality is dwindling down year after year in the US army, probably because it's expensive and they can't justify the cost. At any time, they might have only 40-50% of their plane arsenal ready to depart. This is not huge, though you would rarely have 100% operationality. But again, they have not needed 80% operationality from their planes for a long time. Nobody in Libya was opposing their air superiority, they could fly freely. Nobody was opposing it in Afghanistan, they could fly freely.

Iran though has been building a fully or mostly domestic defense industry under the sanctions, so they have stuff that can target planes and helicopters. So they don't fly freely over Iran.

In terms of logistics, we could look at how they ship all of this stuff around the world (this is where a big navy necessarily comes in), but we also have to state the obvious, now, after 12 paragraphs: the US has stopped producing anything. They outsourced everything to China.

On the ground, this means Lockheed Martin only delivers 50 THAAD interceptor missiles (long-range interception) per year. You would use at least 2 per interception. You might say "but if this is their lifeline in terms of defense, wouldn't you want to make 500 per year instead?" Yes, you would. But they literally can't. They don't have the raw resources or the workforce for it. China controls most rare earths -- rare earths themselves are not rare, what's rare is extracting them from the soil.

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Just because military contractors will sell a desk chair for $10,000 does not mean its worth $10,000, multiply this knowledge with every single purchase done by the US military.