Definitely one of the best non-fiction books I've read this year. I'm going to try and sum up its arguments as best I can, and interject where I feel it's appropriate. So on that note:
GENERAL
-We're living in a strange time in world history, in that empires are a thing of the past, at least hypothetically. Go back hundreds, even thousands of years, and you wouldn't find many people who questioned the logic of empire. However, the 21st century is bizzare in comparison, in that not only are empires shunned, but that the nation-state has taken over as the de facto method of organizing societies.
-There's a myopia among historians on European empires (at least in the West). This isn't to discount their relevance to world history, but you cannot understand global history by casting all other empires to the side, nor can you claim that their legacies don't exist. This has led to a stunted understanding of global history.
-The author doesn't outright state this, but given how language is used, it more or less distinguishes colonialism from imperialism as simply being an empire that doesn't share one cohesive landmass. So for instance, the two largest empires in history (British Empire, Mongol Empire) are both empires whose legacies resonate today, but the British Empire is colonial due to it being maritime, while the Mongol Empire isn't colonial, because it's one cohesive landmass. Speaking personally, I think this is the best distinction I've seen, because otherwise, I've seen people go into knots trying to draw a distinction.
-The world's imperial powers tend to have been on the receiving end of imperial powers.
The book then goes to look at a number of areas. Some of these aspects I found more interesting than others, and I can't sum up all the ideas, but I'll do my best to present them.
UNITED STATES
-There's a question as to whether the US should be considered an empire or not. Personally, I think there's a good argument for the US being an empire (at the very least, its territories aren't all continguous), but that aside, the author highlights how the US is a potential example of an empire that doesn't see itself as an empire, or at least, makes pains to present itself as not being an empire, because this is an era where empire is a dirty word.
-The US is an example of the author's idea that imperialists are often those who were once part of empires, who end up repeating the same patterns. As in, the Founding Fathers explicitly broke away from the British Empire, and declared that all men were created equal, but the country then steadily expanded its territory by force. The Munroe Doctrine was simultaniously anti-imperialistic (in that it didn't want "the Old World" interfering with "the New World"), but also imperialistic, in that it made the Western Hemisphere the US's backyard.
-The section raises the issue as to whether US hegemony has been a net positive or negative for the world in the context of the 20th century. It doesn't pick a side here. Speaking personally, I really can't answer that. There'd be a time where I'd have easily answered negative, but in light of recent events (*cough*Russia*cough), it's easy to see how awful the alternatives can be.
UNITED KINGDOM
I'm not sure why the author focuses specifically on the UK (granted, he's a Singaporean who lives in the UK, so it's his 'home turf' so to speak) as opposed to European empires in general, but considering that the British Empire was the largest empire in history, then it makes sense to include it. So on that note:
-The British Empire's legacy is hotly debated, and insanely polarized. I don't know how true this is, but the author claims that there's basically two camps that go to both extremes, with little discussion in the middle. He cities Nial Fergusson and Sharshi Tharoor, with their books "Empire" and "Inglorious Empire" respectively as examples of the two extremes.
-The British Empire is a weirdo in the history of empires, in that it spread in a hodge-podge manner, beginning with trading ports and growing from there (couldn't you say the same about the Portugese though?) There's no great rolling frontier like you'd find in land-based empires, but even that aside, the British Empire grew up haphazardly, from place to place, and that consequently, there's no single event in British memory that can be called THE point where the empire collapsed (by way of contrast, the author uses Algeria and Indochina as a case of France trying to hold onto its territory).
-The author doesn't say this directly, but Britain does arguably fit into his thesis of "imperialized become imperialists," in that he points out how Britain has experienced waves of invaders over its history, before becoming the heart of an empire rather than the periphery of one (e.g. the Roman Empire).
-Britain's also a case of what the book might call "an imperial hangover," in that it's navigating its place in the world where its status is grossly diminished. Not economically, but more in terms of military prowess - in this sense, Britain invading Iraq is a case of Britain thinking it's more powerful than it really is, with the Suez Canal Crisis being another example. This also (partly) explains Brexit, with Britain seeing itself apart from Europe, and having more of a role in the wider world, regardless as to whether this is actually the truth.
EUROPEAN UNION
Honestly, I found this section the iffiest, and therefore, don't have much to say. Basically, there's the idea of the EU being both an empire and a post-imperial super-state...or something. Honestly, I didn't really follow this train of thought much. If you want my view on things, I don't think the EU can really call itself an empire, nor do I think the EU could be an empire even if it wanted to. By any measure that I've seen, Europe's in no position to be a military superpower given its ageing population, and as some have argued, cultural malaise), but that aside, yeah. I think in the scope of European history, the EU is a net positive, but I don't think it really belongs in a discussion about empire per se.
RUSSIA
-Russia is a classic example of imperialized people becoming imperialists themselves. The author points how the Rus were terrorized by the Mongols, before eventually becoming one of the largest empires in history. It was actually a bit striking to me how you could draw parallels between the UK and Russia - both start off as backwaters, both are subject to waves of invasion, both eventually spread out and dominate large sections of the world, and both suffer what the author calls "imperial hangover," dealing with a reduced presence in the world.
-The author explicitly calls the USSR an empire, and it's a case of paradox/hypocrisy, simultaniously pushing for decolonization of the third world, while also clamping down on its satellite states (though being fair, I'd argue hypocrisy is bread and butter in general, so I wouldn't single the USSR out in this particular case). By extension, the collapse of the USSR marks the end of formal empires in the author's point of view, and what you're left with are either nation states, or empires that don't see themselves as empires (e.g. the United States).
-Russia's actions in Crimea are a case in point of Russia's "imperial hangover" and trying to reclaim lost glory. Bear in mind that this was written before the recent invasion of Ukraine (the breakaway regions notwithstanding), and it's eerile prescient how well the author captures Putin's state of mind).
-Russia is cozying up to China, but it isn't an equal relationship. China has a population of over 1 billion, Russia's population is but a fraction, and incredibly sparse. Overall, the future for Russia doesn't look good.
CHINA
-China's a weird one. It's arguably a case of an empire that became a nation state with its borders fully intact. As in, China the empire and China the country are one and the same.
-China simultaniously calls itself the victim of imperialism (which it was), while also drawing on its own imperial past (which it was). The claim that "China has never attacked another country" is an exercise in semantics, because it's like saying "the Mongols never attacked COUNTRIES" for instance. The author regards this as a toxic combination of nationalism, with China simultaniously playing the role of victim, while also celebrating its imperial legacies.
-Heavy reference is drawn to the Mongol Empire, and how both China and the Mongols have effectively colonized each other over the course of history, with their fortunes changing.
-China and the West (or more specifically, the US) are fundamentally at odds with each other. The author explains it better than I ever could, but basically, the entire conceptions of democracy, and the prioritization of the individual vs. the group are irreconciable.
INDIA
-India has experienced waves of invasions over its history, which have bequeathed their legacies, good and ill.
-Most of the time is spent on examing the Mughal Empire and the British Raj. Time is spent accomodating different viewpoints as to whether the Raj should be considered distinct from India's prior empires or not.
-You're probably aware of the Hinduvata movement in India. The book goes into some detail, kind of claiming that the idea of a 'pure' Hindu India is ahistoric, since India's has had waves of invasions over its history (note that when I say India, I'm referring to the overall landmass, India the country is a new construct).
-Not that the author says this, but India kind of strikes me as an outlier to the theory of "those who are imperialized become imperialists," in that India's experienced waves of invasions, and has had empires rise and fall within its own borders (Sikh Empire, Maratha Empire, etc.), but doesn't seem to have projected its own might out that much. I could be wrong, I think at least one Indian empire spread pretty far south, but meh.
MIDDLE EAST
-The Middle East is dealing with the legacies of both internal and external empires. Internal, in the sense that the ME has had countless empires rise and fall. External, in the context of European colonial empires, which have left artificial borders. Both of these legacies have their own issues with them - for instance, ISIS made a point of demolishing the border between Iraq and Iran, but was also trying to claim a linneage of the Islamic caliphates.
-Quite a bit of time is spent on the Ottoman Empire. The author seems fairly approving of its record, such as the millet system, though doesn't ignore the Armenian Genocide. The basic undercurrent seems to be that empires are (or were) better than nation-states of accomodating different ethnic groups. That may be true, but I'm not sure if that should be used as a case for bringing back empires (not that the author is making that claim).
-Time is spent on the Iran-Saudi rivalry. These two bodies aren't necessarily empires, but are drawing on history for their competing claims over the ME.
-Again, interjecting, I think the ME is a case where it's too simple to say "the imperialized become imperialists," because not only is the ME a broad swathe of land, but there's no distinct chronology. The ME has birthed empires that have spread from beyond the region (Ummayad Caliphate, Ottoman Empire, etc.), but has also been the subject of empires coming 'into' it (Alexander's empire, the Mongols, European empires, etc.)
AFRICA
-Brief mention is given to the Ghana, Mali, and Songhai Empires, as well as Great Zimbabwe (does that count as an empire?) Honestly, the first three are examples of repeatedly observed phenomena in history, of empires rising and falling over the same patch of land.
-The author makes specific reference to the Trans-Atlantic Slave Trade as being unique due to depopulating West Africa. Personally, if you want to call the TSA unique, this seems like the weakest of differentials, because if depopulation is the only issue, then that's fairly common in slaving, ranging from Easter Island to the so-called Harvesting of the Steppe of eastern Europe.
-If you want to ask why Africa is the way it is, and you say "colonialism" or "tribalism," then the author takes the view that the answer is "both," and that neither extreme can generate a full picture. On one hand, Africa has the legacy of artificial borders and weak national identity, but on the other, that doesn't excuse the actions of African elites, nor does it deal with tribal rivalries hindering development. Personally, I think this section is ill-served by not even mentioning Ethiopia, since it never came under European control in any real sense.
-If you're looking for an optimistic outlook on Africa here, you won't find it in the book. It does make me wonder, again, hypothetically, if Africa is another break from the whole "imperialized become imperialists" thesis, because as of typing this, there's no real chance of that happening. The continent is fractured, the AU is limp, Pan-Africanism is dead, and even if you factor out all the social issues, sub-Saharan Africa has a number of disadvantages. Not all of these claims are the author's own, but basically, the prognosis is the same. And that's not even factoring issues such as climate change. :(
CONCLUSION
Overall, the book was a solid read. There's a chance I've mischaracterized some of the author's arguments, and obviously I don't agree with everything, but overall, it's solid writing.