I don't think that's true. Warfare in general has advanced quite a bit, and expertise exists in many places. You can't really hold an empire with just force these days. The US would crumble under an attempt to take over anything more than North America.
The approach of the modern superpower is much more effective: decentralized power but overall alignment. There's no coincidence to America's approach. It isn't ideological. It's pragmatic.
1) Much of that expertise has come directly from US involvement. See: Western Europe, Japan, Korea.
2) Much of the other expertise developed specifically because of US non-involvement. See: the Soviet Union and the states it influenced and supported. There were even plans to attack/nuke the Soviets, who were the only real rival post-WW2.
Had the U.S. wished to, it could have easily formed an imperial state following WW2. That would have been in-line with essentially every large state in human history. Thankfully, they didn't, and the world is better off for it.
I actually think America couldn’t have formed an imperial super state. That is really, really hard. Even the pre-eminent superpower of the pre-World-War Era, the United Kingdom, was having her empire fray and tatter before the Wars. The forces at play are too powerful. Rapid collapse would have followed. And that’s even assuming we could have beaten the USSR by 1949 (First Lightning), when opposition nukes enter the picture and the game changes again.
The tools of modern warfare are too widespread and easy to adopt. You can’t do it anymore, the old school empire is just as dead as old school fortresses. Technology has broken it.
I have friends who served in Afghanistan. I’m not unfamiliar with how powerful the US military is. But even that power is too weak.
America’s approach is way more resilient and the control exerted is much easier to retain.
Besides the technology of weapons, a big change here is that the economics of running an empire was completely changed by industrialization.
Every empire since the year dot captured land, with peasants farming it, who could be taxed, to pay the soldiers, to capture land... that was the business model. A little profit was left over to pay for cities, palaces, pyramids, etc.
This flow of money from country to towns was reversed by the industrial revolution. Within almost every country now, the cities are taxed to pay the rural voter base (or to pay off the rural might-be-revolutionaries, in places with less voting).
Not the only thing going on, of course -- changes in weapons matter to the balance, as mentioned. So does the rise of literacy & nationalism, which made the idea of rulers who are too distant (by ethnicity, language, or religion) from the population base (as they were in empires, almost by definition) into a rallying point. And thus raised the cost of holding land.
You're right that you can't hold an empire with only force, but I would argue that was just as true during the days of the Roman empire as it is today.
Sure, the Romans used (for the time) overwhelmingly advanced technology and amazingly brutal tactics to conquer Gaul and Britain, but they maintained their hold over those territories by culturally assimilating the native populations. Gallic and Britanic Romans spoke Latin, wore Roman dress, and even followed Roman sanitary customs -- there's a reason there's a town called Bath in southwestern England.
That culture continued to exert influence long after Rome fell as a central military power.
>Roman Empire, British Empire, Ottoman Empire, Japanese Empire, Mughal Empire
Imagine, for a moment, that each one of these had nukes. What's the probability they'd use them to make a point as an aggressor/opressor? Even risking MAD?
Probably the same, with the same knowledge available. If you just gave a Roman Emperor unlimited use of nuclear weapons, sure, he might go ham. He didn’t even know the extent of the world. But you don’t get nukes without awareness of what a nuke is.
Given everything, I think the highest likelihood outcome is that they behave similarly.
The US definitely would have used nukes, especially in Vietnam - I believe that the reason the US President is the one with the nuclear suitcase and who it the end decision maker (probably something more people are nervous about now) is because when Truman learned about the nuke project (I'm not sure he had been aware of them until he became president) and used them he was afraid the military would use even more. Former SecDef Perry does a lot around nuclear nonproliferation and has good free courses, a podcast, and a book https://www.wjperryproject.org/
The approach of the modern superpower is much more effective: decentralized power but overall alignment. There's no coincidence to America's approach. It isn't ideological. It's pragmatic.