At the dawn of the 20th century, the United States was a power hiding behind oceans. In the first decades of the 21st century, it is the world’s only hyperpower, able to project its military and cultural influence to every corner of the world. This fascinating book – at least, it’s fascinating for those with an interest in the political and sociological history of America – tells how America made the decisive turn towards engagement with the outside world.

It may be hard to realise now, but through most of its history, isolationism has been the strongest strand to America’s foreign policy. Its founders and first generations of immigrants crossed the sea to escape the wars and persecutions – political and religious – of the Old World. Having found a home in the New World, they had no wish to engage in the wars of their old homes. So when the First World War broke out, America remained neutral. Not only did this keep it out of the war, neutrality brought huge profits in its wake, as American goods and products found ready markets among all the combatants.

But such blood profits sat uneasily on American consciences, bought as they were in the immolation of a continent that many Americans still thought of as home. For none was this problem more acute than for German-Americans. Where did their loyalty lie? At first, they pushed for continued American neutrality. But as the war continued and incidents such as the sinking of the Lusitania increased anti-German feeling, such a position became increasingly untenable. War was coming. And German-Americans, in common with Irish-Americans, Italian-Americans and the other national groups, came to the one, common conclusion: they were Americans before they were anything else. Thus, the First World War killed off the 19th-century American experiment in multiculturalism (played out in a multitude of national-language newspapers and societies) and ushered in a new consciousness of what it was to be American.

Neiberg tells the story of this profound change through an encyclopaedic knowledge of the time, ranging from popular songs, through speeches and newspaper articles, to the letters of people ranging from Theodore Roosevelt to ordinary mothers contemplating the possibility of their sons being called up. It’s a great piece of scholarship – but only bother with reading it if you’re interested in the subject.