Tom Devine: looking back on September 18, three months on

Why did the Anglo-Scottish Union come to an end in 2014 after more than 300 years? There was not the slightest clue in the middle of last century that Scotland would become an independent state merely a generation or so later. In the 1950s and early 1960s, the Union was totally secure. The experience of the two nations in a total war between 1939 and 1945 had given a collective cohesion to the relationship. The development of the post-war welfare state provided a new anchor as the British Empire, that traditional bastion of the Union, began to disintegrate. The increasing Soviet threat meant there still remained an enemy "other", which ensured the need for a continued strong defence within Britain.

Moreover, in the 1950s, the SNP was a small protest group rather than a mainstream political party. At the time, Unionism was dominant in ­Scotland, confirmed by the stunning and overwhelming victory of the Scottish Unionists (not Conservatives until 1965) at the 1955 General Election. In 1950, even the Labour Party gave up its long-term interest in achieving Scottish Home Rule.

The rise of a more successful and effective SNP is often dated from the Hamilton by-election victory of 1967 by Winnie Ewing. But the failed devolution referendum of 1979 suggested that many Scots still had doubts, even about limited reform of the Union state.

The real decline of the appeal of the Union began seriously in the 1980s. At least four factors can be identified as significant.

First, the de-industrialisation of Scotland in the 1980s and its adverse social effects were of paramount importance. Between the late 1970s and 1987, Scotland lost one-third of its manufacturing capacity. Rightly or wrongly, this catastrophe was blamed then and ever since on the hard-nosed economic policies of the Conservative governments of Margaret Thatcher, despite the fact these great industries of the past suffered also from grievous internal weaknesses and were exposed to increasingly formidable competition in global markets. Tory enforcement of the poll tax, denationalisation of industries and welfare reform abruptly ended the tradition since the later 18th century of a sensitive and accommodating balance between Scottish concerns and broader British interests within the Union. From that time on the Anglo-Scottish relationship was destabilised, though not yet in peril.

A post-industrial economy did emerge from the crisis by the later 1990s. But the devastation caused to many mining and manufacturing communities during the previous decade lingered on. They became scarred by long-term unemployment, poor health, unacceptable levels of male mortality, and drug and alcohol abuse. These marginalised people and their families were eventually recruited in large numbers to vote for independence during the recent referendum campaign.

The other crucial transformational effect was the electoral destruction of the premier political champion of the Union as the Conservative vote collapsed and Scotland became a Tory-free zone by the end of the 20th century.

Second, the 1960s ushered in a process of dramatic secularisation in ­Scottish society. Between that decade and today, the Church of Scotland lost around two-thirds of its membership, with the haemorrhage especially acute among young adults. This development, combined with working-class alienation from Conservatism in the 1980s, destroyed another of the Union's most formidable supports, the sectarian voting patterns which had existed in Scotland from the 1920s. Working-class Protestants tended to vote overwhelmingly for Scottish Unionism while their Catholic counterparts, mainly of Irish descent, were equally loyal to the Labour Party. Partly for this reason the electoral space for a popular nationalist movement was very limited. But by 1990, working-class Conservatism, based on loyalty to Protestantism and hostility to the Catholic other, was in its death throes. Sectarianism had not disappeared but the decay of connections with the Church of Scotland and popular disgust with Conservative policies in the 1980s broke the relationship forever. The SNP were eventually able to exploit the resulting electoral space with considerable success, firstly in the northeast of Scotland, and then eventually in parts of the central belt.

Third, the transformation of the SNP in the last few decades has also been of critical significance in the forging of a powerful political threat to the Union. The metamorphosis from ethnic to civic nationalism, followed by a shift to left-of-centre, especially in the social sphere, ensured the party began to have wider appeal than at any time in its history.

But the SNP still failed to make any long-term breakthrough in Westminster elections. Its saviour was the opening of the Scottish Parliament in 1999, ironically delivered by its mortal foe, the Labour Party. The Nationalists then used this opportunity to great effect, especially in the last two Holyrood elections, helped by disillusion in many communities with the drive of Blairite New Labour towards the political centre and the opposition of many Scots to the Iraq war.

The perceived mediocrity of several Labour ministers and MSPs in the new Parliament also helped. Hugely experienced Scots Labour politicians, the so-called "big beasts", decided to stay in Westminster and advance their careers there. Only the late Donald Dewar came to Holyrood. The party which most Scots had supported since the collapse of Toryism now appeared to suffer from a chronic deficit of talent and sclerotic organisation at the grass roots in Scotland. This became even more apparent when the two SNP governments, ably led by Alex Salmond as minority and majority administrations, were considered by impartial commentators to have delivered competent governance of the country.

Fourth, there is the issue of identity. The dual or hybrid identity of Britishness and Scottishness emerged in the later 18th century and has shown resilience and flexibility since then. In the 1950s, Britishness was in the ascendancy. Since the 1990s it has been Scottishness. A greater sense of national confidence and a veritable explosion in Scottish literature, history-writing, music and culture more generally over the last few decades have fuelled this dynamic. An increasingly assured Scottish identity did not cause independence. But it was by any measure a necessary precondition for that achievement.

Finally, the referendum campaign itself deserves some scrutiny. David Cameron, the former Prime Minister, scored a spectacular and historic own goal by steadfastly refusing to have three questions, including devo max, on the ballot paper. If he had done so, there would have been an overwhelming victory for the option of enhanced devolution in the ­referendum. Organising an effective campaign led by three different political parties was always going to be challenging. Commentators at the time frequently suggested there was too much negativism and not enough attempt to provide a more positive vision of a future Union between England and Scotland.

Then came the polls of September 7 and 9, 2014 confirming that the result of the referendum was too close to call and even indicating a momentum in favour of Yes. This resulted in panic among the Unionist parties and the offer, now seen as too little too late, of an enhanced package of devolution measures.

Like the threats by Unionist spokesmen from both Conservative and Labour parties, this proved counterproductive. The carrot proved as unattractive as the stick. No senior politician at Westminster had the ability, courage or imagination to see that federalism was the only option to pursue to save the Union.

By contrast, the Yes campaign not only achieved its objective but was seen to have been a much more effective force throughout. Its message was more upbeat and visionary, and near the end emphasised that retention of the Union was in itself a gamble by playing up potential threats to the NHS and other public services, whether real or exaggerated. This tactic proved in the final analysis to convince many wavering working-class voters in particular, not least in the more disadvantaged parts of the big cities. The young were also stimulated by a movement which seemed so much more "cool" and vibrant than the opposition.

Days before the decisive vote, three of the country's major banks (two of which had been bailed out by Scottish taxpayers), a couple of large retailers and the Standard Life Insurance Company, threatened Scotland with dire consequences in the event of a Yes vote. This was regarded by the electorate as brazen interference by the forces of market capitalism in the democratic decision processes of the Scottish people. The intervention proved counter-productive and after the referendum result many Scottish consumers exacted revenge on these corporations by withdrawing business from them.

This is a personal view, written soon after the historic event, by one historian about the main reasons why a union of two ancient nations which had endured for more than 300 years is now no more. History is by definition a subject based on interpretation and so, without a doubt, the reasons for the collapse of the British Union will be argued over by future ­scholars for many generations to come.