In both British and American history, fervent imperialism has always coexisted with bouts of fierce isolationism.

From the very beginning, Americans have exhibited a taste for expansion, an appetite for empire. One of the fundamental reasons for this is very clear. Like every other western empire that has ever existed, Americans may claim to have inherited the mantle of ancient Rome.

From the American Revolution right up to the Second World War, the U.S. was more likely to provoke suspicion among members of the British establishment than deferential approval. It was seen - with good cause - not just as a potential rival for empire, but also as dangerously egalitarian, worryingly innovatory, and excessively democratic.

Empire in the past was always a far harsher and much more accident-prone business than conventional history books imply. And the costs of these overseas invasions were borne not just by those on the receiving end but - frequently - by ordinary, vulnerable people among or associated with the invaders.

America is the proud possessor of the oldest extant written constitution in the world, which was for its time - 1787 - a highly innovative and important document.

I write to relieve an intellectual itch. I stumble across a hitherto neglected set of events, transformations, characters, or source materials from the past, and they nag at me until I make sense of them in words. But I also write to seduce and to make my readers think.

Many Britons who backed Brexit believed - and believe still - that a U.K. 'freed' from 'Europe' would be able to recover and re-establish its historic destiny as an independent global trading nation.

A break-up of the U.K. would affect the deployment and strength of its armed forces and play havoc with the ownership of its overseas consulships and embassies.

In the U.S. - and elsewhere - successful parties need a storyline that voters can relate to, an intelligible plot of some sort, especially now that so many older, formal ideologies have lost force. For proof of this, one has only to look at Margaret Thatcher's career and ideas.

Even leaving aside its military bases, America's influence on the domestic ordering of British life has been enormous, though sometimes unrecognised.

To be sure, political unions between European countries have often failed in the past, but usually only after relatively brief periods. Denmark and Iceland separated after 130 years; the unions between Spain and Portugal and between Sweden and Norway each lasted less than a century.

High-level political wives are by no means new. In the 18th and 19th centuries, when patricians dominated British political life, it was common for politicians' spouses to play an active political role.

Hillary Clinton is tough, clever, and formidably well briefed, and has been politically ambitious all her adult life.

Many Americans remain very interested in royal goings-on in general, and not just because of their soap-opera appeal. To a greater degree than any other polity, Britain functions as Americans' defining 'other.'

British prime ministers and prime ministers' spouses and children are together becoming ever more like first families. They need to be given sufficient resources and personnel to enable them to carry out their shifting roles efficiently, decently, and safely.

Before they became Americans, most white inhabitants of the 13 colonies considered themselves British. It was predictable, therefore, that they would lust after empire, because this was exactly what their counterparts on the other side of the Atlantic also did.

Once you know how completely and suddenly the earth can open up at your feet and the worst can happen, it also, paradoxically, leaves you more afraid of everything else.

Look at how the British covered India with railroads, and it is easy to view them as modernisers. Look, however, at the abysmal levels of mass illiteracy in the subcontinent they left behind in 1947, and they appear rather differently.

In Britain, British history is naturally a mainstream subject. Step outside your own narrow specialism, and you can find yourself treading on someone else's toes. But in America, British history is an eccentric, minority pursuit, and while this can be intellectually isolating, it also permits extraordinary freedom.

I was born and spent my first five years in Chester, an ancient city that retains some of its Roman walls and fortifications and contains a great medieval cathedral, as well as Tudor, Stuart and early 19th century architecture. Visiting these things was free, and my parents - who had little money - made the most of this.

Traditionally, royal females who have not had the luck to become queens regnant have been granted very limited roles. They have been expected to look pretty, be discreet, do charitable good deeds, and - if married to princes or kings - be quietly supportive and, above all, fertile.

Globalisation is not remotely new; it has been occurring, at differing rates and with differing degrees of scale, for centuries.

The so-called Boer War advertised British vulnerabilities, and these were confirmed by the Irish rising of 1916 and the subsequent creation of the Irish Free State, blows that attracted the notice and attention of colonial dissidents in Asia and Africa.

The American revolution not only cost Britain the 13 colonies but also forced it to rethink the slave trade and slavery, and influenced its power relations in Asia and the Pacific.