Mike Elrick thought he knew all about blood, sweat and tears as he sat slumped in Holyrood Park after completing last Sunday's Great Edinburgh Run. Then he switched on his mobile phone.

As it started to beep like the heart monitor of a patient going into cardiac arrest, he was given a painful reminder of what it is to be Wendy Alexander's principal policy adviser.

Unknown to him, his boss had just gone on live television and announced she was scrapping 100 years of Labour unionist tradition by committing the party to support for a referendum on Scottish independence. In three short words - “bring it on” - she had, unilaterally, changed the Scottish political landscape.

In the days that followed she would go on to publicly humiliate the prime minister - her lifelong political mentor - alienate her unionist party colleagues who had, in good faith, signed up to her putative constitutional commission, bring the Scottish Labour party to its knees and hand the Scottish National party, her sworn enemy, a propaganda victory that Alex Salmond could not have hoped for in his wildest dreams.

This morning, after her most bruising week in a career that has, at times, resembled that of a heavyweight boxer, she is assessing the remnants of her reputation and facing a growing clamour for her resignation. More significantly, because of her misjudgment and political naivety, Scotland is now considering a very different future from that envisaged this time last week. She committed the country to an extended period of uncertainty, culminating in a constitutional referendum in 2011. The Conservatives and the Liberal Democrats can do nothing but participate, however reluctantly, in a process they didn't want.

In the fervour of last week's exchanges at Westminster and Holyrood, what was not explained was why she made the announcement, who knew she was going to make it and why nobody did anything to stop her.

Alexander's shambolic policy change coincided with a combination of circumstances. Labour had just lost 331 seats in council elections in England and Wales, while Boris Johnson had just become London mayor, amid some of Labour's worst polling figures for 40 years.

North of the border, the SNP was enjoying record polling figures, suggesting it would gain 15 extra seats at Holyrood in the next election in 2011. At the same time, a YouGov poll suggested support for independence would be below 20%, provided people were asked if they wanted Scotland to break away from the rest of the UK.

The day after the disastrous elections, Alexander and Brown spoke on the telephone, according to senior Labour sources. Alexander suggested that perhaps the time was right to change the party's long-held opposition to a referendum on independence. If the poll was held sooner rather than later, with support for separatism so low, the electorate would almost certainly vote against.

It was a risky strategy that might be criticised for playing fast and loose with the Union but if it worked, it would shoot the nationalist fox. With its raison d'être defeated, the SNP would lose purpose, authority and, ultimately, popularity.

While Brown did not confirm his support for such a change of tack, neither did he pour cold water on the idea, which Alexander took to mean he was open to persuasion.

Against this backdrop, she went on BBC Scotland's Politics Show. Referring to Salmond's planned referendum bill in 2010, she said: “I don't fear the verdict of the Scottish people. Bring it on.” Even with the quote zinging across the news wires, Alexander still had enough wriggle room to downplay it as a reference merely to the referendum bill, not the referendum itself.

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