So the United States Army has recently been thinking about megacities, that growing cluster of urban giants containing more than 10 million souls. Here are its main conclusions:

It's basically inevitable that it'll send troops into a really big city one day;

It doesn't think that city will be New York, but it might need to invade Bangkok or Lagos;

It doesn't currently have the first clue how to do this;

It thinks it can probably learn how to re-establish order, but – and this is a reassuring bit – "this order may not resemble previous conditions".

So, to sum up, everything's going to be fine.

This cheery vision of the future is to be found in a report from the Chief of Staff's Strategic Studies Group (SSG): a sort of internal military think tank which conducts "independent, unconventional, and revolutionary research” into things you can do if you have the world’s largest standing army. When one thinks of the urban studies community, this is probably not the first group that springs to mind.

“It is inevitable that at some point the army will be asked to operate in a megacity, and currently the army is ill-prepared to do so”

– Megacities and the United States Army: Preparing for an Uncertain Future, p3

But the SSG gives a number of reasons why it’s decided to start thinking about this topic. The world’s growing cluster of megacities contain a growing share of the world's population; they produce a growing share of its GDP; and they play a critical role in key resource markets ("Some megacities," the report says, "will be conduits for access to critical natural resources like petroleum"). So, the implication runs, war will get one of them eventually, by the law of averages alone.

Actually, the report gives a more tangible explanation for its existence, too: the megacities of the developed world are unusually prone to unrest. They grow quickly, in unplanned ways, and often contain areas (slums, favelas) where government control is tenuous, at best. They bring people of different religions, ethnicities or wealth levels into close proximity, which can generate tension. They're well connected to global transport networks, and are easy to get in and out of.

The result of all this is that “operating from megacities allows hostile actors relative freedom of maneuver as they blend in with the local population”. Or, to put it another way: “We think bad people are there, and we don't know how to find them.”

"Not knowing stuff" is actually the main theme of this report. Historically, when the US has needed to conquer a foreign city, its strategy has basically been (I simplify) to surround the thing and walk inwards until their troops control it. You can't do that with a city of 10 million people: it's just too big. The physical infrastructure is rubbish, so the roads are too narrow to get a tank through; but the technological one is brilliant, so the guys on the other side can use their mobiles to co-ordinate resistance. And the current urban training environment, pictured on p9, leaves rather a lot to be desired:

As a result, “it is inevitable that at some point the army will be asked to operate in a megacity, and currently the army is ill-prepared to do so”. It’s the SSG’s job to make sure the army isn’t ill-prepared for anything; ergo, it’s having a think.

If your job is to ensure that the US military is ready for any war it may one day have to fight, then all this no doubt makes sense. If you’re a mere citizen, however, its conclusions are just a tad worrying.

For one thing, consider which cities it talks about operating within. "Highly integrated" cities – good infrastructure, good government, some control over who goes in and out – are “anti-fragile”, can look after their own affairs, and are therefore fine. The section on New York reads like a cross between a tourist brochure and a panegyric to the fire fighters of 9/11 ("a rich history of assimilation and integration.... supporting infrastructure is not simply rebuilt, it is rebuilt stronger and able to withstand more"). New York City will not face military occupation any time soon.

Work your way down the development table, though, and the attitude changes. Bangkok has decent infrastructure, yes – but it also has soaring income inequality and a lot of coups, so "it is not unreasonable" to think the US army may find itself there on "counter terrorism operations" one day. (You'll be pleased to hear its extensive canal network will help on that score.)

Lagos, meanwhile, has a soaring population, enormous slums, and is in an economically critical country. All this means that foreign assistance could be required one day – “and, considering America's significant economic stake in Nigeria, some US military assistance might be offered".

"Offered" is a key word here: the language right through the report is that of invitation, not invasion. Now to be fair, the vast majority of megacities are in states allied to the US, and in the current climate it'd be more than a little inflammatory to publish a plan for occupying Moscow. Nonetheless, it leaves you with the odd impression that the Americans are cheerfully envisioning how they might go about occupying the biggest cities in friendly countries. Why Lagos and not Kinshasha? Why Rio, not Bogota?

"By 2030, there will be 37 cities across the world that are 200-400% larger than Baghdad"

– Megacities and the United States Army: Preparing for an Uncertain Future, p8

There’s one more worrying aspect to all this: the report gives very little indication of what occupying one of these places would actually mean. The Army explains why it might want to operate in a major city. It explains the barriers to doing so. It contrasts the cities it might invade with those that it probably wouldn't. But at no point does it give us the first clue what a US Army presence in Lagos might actually look like. The comparisons with Rio and so on talk about policing activities; but other parts of the report suggest total war. Are we talking ground troops here? Tanks? Air strikes? It's not clear – and in that ignorance lurks terrors.

The US army isn't planning to invade any of these places, of course. It just wants to know it can. It has a responsibility, the report says, "to prepare itself to provide the right tool to the Nation when contingencies arise".

Before the Iraq War, the US army failed to come up with a viable strategy for winning over Baghdad. It won’t make that mistake again. When war comes to Lagos, it intends to be ready.