Its consequences for the British archipelago and Britain’s place in the world will be incalculable for many years to come. A third of the UK land mass will immediately cease to be British territory. “South Britain” beckons as the presumptive name for the remainder of the UK. Even if the no campaign manages to achieve a narrow victory, the crisis in the union will continue unabated. The evidence to date suggests that Westminster politicians do not yet possess the imaginative capacity to deal with it.

All this presents a major intellectual challenge for the historian. Little more than a generation ago, in the 1950s and early 60s, the union could not have been more secure.

The memory of the collective British sacrifice of the second world war lived on for the postwar generation in comics, books and films. The empire started to dissolve with the independence of India in 1947. Yet, contemporaneously, the welfare state was established and soon became the new sheet anchor of the Anglo-Scottish union. Nationalisation of key industries further strengthened the idea of a British-wide collective economic enterprise.

Yet all this can be seen in retrospect as the quiet before the storm. Winnie Ewing’s surprise victory for the SNP in 1967 was a small but significant portent of what was to come. The crucial historic importance for Scotland of maintaining free access to English markets ceased to be of such importance when the UK entered the common market.

A shared English and Scottish commitment to Protestantism in the past had provided much of the ideological glue of union. This is no longer so in the age of secularisation.  he two political parties most committed to the union left little space for the growth of nationalism. Moreover, Catholics had at that time a profound suspicion of the SNP, believing it to be dominated by Presbyterians, as well as having a few notorious bigots in its senior ranks.

The experience of Scotland in the 1980s lost nearly a third of its manufacturing capacity. The great heavy industries that had made Scotland’s global economic reputation over more than a century disappeared in a matter of a few years. A post-industrial economy did emerge in the 1990s, but the crisis left behind a legacy of social dislocation in many working class communities and created a political agenda north of the border in marked contrast to that of the south of England.

Equally fundamentally, state involvement and public spending became even more important to many Scots,  It is also the root of those Scottish political attitudes that seem to favour Scandinavian-type social policies, and which strongly oppose neoliberal market economics, associated in the public mind with the alien ideology imposed during the Thatcher years. Sean Connery at the opening of the Scottish parliament in 1999. ‘The parliament eventually became the vehicle for a transformed SNP.’ Photograph: UPPA Ltd.

The foundation of the Scottish parliament in 1999 and the referendum were not directly linked causally. But the parliament did eventually become the vehicle for a transformed SNP to gain political power and then trigger the referendum process.

There may well be a no vote on Thursday. But a victory for unionism will be far from decisive or definitive. Nearly half the Scottish electorate will almost certainly vote yes and may not be easily satisfied by post-referendum  concessions that are also likely to further fuel resentments south of the border. If the yes campaign wins, Britain will never be the same. Three centuries and more of political union between England and Scotland will be consigned to history. It’s the possible end of an old political union rightly thought by many Scots to be no longer fit for purpose.