After a famous win, the chance to restore the United Kingdom

David Cameron's election victory has sprung one of the biggest surprises of the modern British political era. After a bare-knuckle campaign dominated by predictions of hung parliaments and electoral chaos, the Conservative party has been returned to power with an absolute majority of 12 seats over the other parties at Westminster.

Not only does the result leave Mr Cameron in Number 10 Downing Street; he can govern without the support of coalition partners. It is an outcome few pollsters or pundits predicted - even as the polling stations closed.

The prime minister has won a very slim majority by UK standards. Past prime ministers such as Margaret Thatcher and Tony Blair enjoyed three-figure majorities. But Mr Cameron's achievement should be welcomed. It banishes the prospect of paralysis at the heart of government.

In the run-up to polling day, British business was unnerved by the likelihood that the election would fail to produce a clear winner. Bosses fretted about the possibility of weeks of post-election haggling between the parties. Now, that threat has receded. Markets rose as businesses breathed an audible sigh of relief.

Securing the first Tory majority in nearly a quarter of a century is a personal triumph for Mr Cameron. Not since Sir Anthony Eden in 1955 has a sitting Conservative prime minister pushed up the party's vote share at a second election. Great swathes of the map - especially across Liberal Democrat redoubts in southwest England - have been coloured blue.

True, the Conservative campaign was ungainly and sometimes muddled. Nonetheless, it was ruthlessly effective. The prime minister hammered home the message that Labour could not be trusted with Britain's unfinished economic recovery. Despite the absence of a "feel good" factor, he was rewarded for the progress made in the past five years by the Conservative-Liberal Democrat coalition. Faced with the opposition's prospectus - one based on distrust of business and wealth creation - voters rightly concluded that continuity was the better choice.

As the Conservatives revel in their unexpected triumph, they should recognise that victory opens a new and uncertain chapter for the United Kingdom. The stunning success of the Scottish National party, taking 56 of the 59 seats north of the border, threatens to reopen questions about Scotland's place in the union thought closed after last September's referendum vote against independence.

Having played the "Scottish card" during the election, warning about a possible leftist alliance between Labour and the SNP, Mr Cameron could see the mood curdle into sullen nationalism. The union risks being rent asunder by the collision between two incompatible forces: rampant, spendthrift Scottish separatism and a Tory government committed to austerity at Westminster. This would be a true winner's curse.

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These are not themselves new challenges. What has changed is that Mr Cameron can address them with enhanced authority. After long-running speculation that senior colleagues might bid to topple him as leader, his position is secure for now.

In the wider political battle, the stage has suddenly cleared following the quick-fire resignations of Britain's other main party leaders. Mr Miliband's exit leaves his party languishing after its worst performance at the polls since Michael Foot's "suicide note" election of 1983. The task of reconstruction will be especially arduous given the obliteration of Labour's top ranks in Scotland and the shock loss of shadow chancellor, Ed Balls. Ominously, the europhobic UK Independence party has increased its share of the vote. However, it has won only one seat and its leader Nigel Farage, once seen as a charismatic challenger to Mr Cameron's Tories, has been toppled after failing to secure a place in the House of Commons.

The one poignant loss is that of the Lib Dem leader, Nick Clegg, after presiding over a "catastrophic" collapse in his party's support. Reduced to a rump of just eight seats in parliament, the Lib Dems can feel harshly dealt with by voters for their public-spirited decision to enter a coalition with the Tories in 2010. If there is a bright spot it is that Mr Clegg remains in parliament. He still has a significant contribution to make to British public life.

In his hour of victory, Mr Cameron cast himself and his party as a unifying force. He returns to Number 10 leading a country fragmented by narrow nationalism and still bruised by the inequalities in income and living standards exposed by the financial crisis. In his acceptance speech he stressed his wish for the Tories "to reclaim a mantle that we should never have lost, the mantle of one nation, one United Kingdom". These are important sentiments, and should be acted upon.

On the future of the union itself, Mr Cameron should now work towards a new constitutional arrangement that not only fulfils the pledges given to the Scots after last year's referendum but distributes power fairly across the four nations of the UK. The Conservatives' fixation with "English votes for English laws" should be curbed. There should be no crude delegitimising of the SNP.

On Europe, Mr Cameron embarks on his second term with the advantage of having the attention of other EU leaders focused upon him. They now know that a British referendum on membership is inevitable. In view of this, and the shortness of his self-imposed timetable, Mr Cameron should set out swiftly what he wishes to achieve from a negotiation and reach out imaginatively to the partners he needs to convince. He must abandon the temptation to appease the eurosceptic wing of his party with promises of reforms he knows to be beyond reach.

Mr Cameron's relationship with his backbench MPs has never been easy. This may change now he has been anointed as an election winner, an accolade that eluded him in 2010. But with a wafer-thin majority - smaller even than John Major's troubled administration in 1992 - he will have to manage a bloc of Tory MPs whose obsession with Europe and flirtation with English nationalism will not easily be contained. This will require a demonstration of leadership and statesmanship that was rarely visible in Mr Cameron's first term.

Mr Cameron has scored a famous victory. He enters his second term with few rivals and much room for manoeuvre. Having stated publicly that he will not stand for office again, he is liberated from the tyranny of the electoral cycle. Like US President Barack Obama after his 2012 victory, he has scope to define his own legacy.

The prime minister's stock of political capital will never be higher than it is now. This is no time to take a breather. Historians will not remember him for election victories, however improbable. He will be judged ultimately on how he resolves the great questions that overhang the UK and its place in the world.

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