London (CNN)No one, probably not even David Cameron himself, thought it would end like this.
The British Prime Minister's stunning election victory on Thursday defied pollsters and pundits predicting a fraught new age of fragile coalition governments and lurching political instability in London.
His triumph was so total and so unexpected that three rival party leaders had fallen on their swords by lunchtime Friday and polling companies had launched a probe on what went wrong.
Britain's global partners, meanwhile, breathed sighs of relief that Cameron's victory, and slim parliamentary majority of 12 seats, averted political chaos in London that could have further eroded Britain's already diminished global standing.
Still, once the celebrations die down, Cameron will face immense challenges over E.U. membership and what to do about a rebellious Scotland that will dictate the place Britain, with whom America has a "special relationship," will occupy on the world stage. And the outcome of those struggles could raise questions about Britain's international leadership.
In the short term, Cameron's capture of an outright majority for his second term enhances his stature among fellow world leaders and offers a chance to redefine a reputation of being someone stronger on slick presentation than substance and who is not yet known as a dominant leader with a clear sense of direction.
Conservatives liberated from their coalition partner
His liberation from his centrist coalition partner the Liberal Democrats -- who always clipped his wings -- will also allow him to dole out more government posts as patronage to keep his own troops in line.
"There is some potential for some form of leadership or some form of policy coherence going forward," said Nicholas Dungan, a senior fellow at the Atlantic Council.
"The trouble is that policy coherence may be something very new from David Cameron. We haven't had much or it in the last five years -- there is a big question about his political maturity."
Cameron looked suitably jubilant -- and a little smug -- as he walked up Downing Street with his wife Samantha on Friday morning after his surprise election win.
But he must quickly get to work on two huge, even existential, challenges for the United Kingdom, that the election has only made more acute: Scotland and the EU.
He'll face a constant battle to ensure that the U.K. political class does not become inward-looking and consumed with Britain's survival as a unified state or the potentially explosive debate over Europe.
Cameron has promised to renegotiate the terms of Britain's membership with the EU, to mollify many Britons who believe that too much sovereignty has ebbed away to Brussels. After that, he has pledged to hold a referendum by 2017 on whether to leave the bloc. The outcome of that vote is hard to predict.
"We will now face five years where questions will arise about the future of our union, about whether or not we can stay as a member of the EU," Ed Balls, the Labour Party's finance spokesman, who lost his seat in parliament, told CNN affiliate ITN.
Sir Martin Sorrell, CEO of WPP PLC, one of Britain's leading businessmen, told CNN that the Conservative Party's victory, which was welcomed by the markets, was a good sign for businesses big and small. But he cautioned that the focus turned to the EU referendum "the moment those exit polls came out last night."
Though Cameron doesn't have to worry about keeping coalition partners in line any more -- the Liberal Democrats were almost wiped out by the election -- his political problems are by no means over.
Cameron will likely spend considerable political energy trying to keep the troublesome Euro-skeptic wing of his Conservative Party in check.
He will try to avoid the fate of former Conservative premier John Major 1992, who saw the promise of his own surprise election win shattered by fighting about Europe among troublesome party lawmakers he later branded "bastards."
"In some ways, (Cameron) was protected by the Lib Dems as the leader of a coalition government. It protected him from the right wing of his party," Mike Finn, professor of history at Liverpool Hope University, told CNN.
"He is going to have to face up to some of his demands from his right that previously he has not had to accede to."
Nigel Farage of UKIP defeated
Cameron received some good news early Friday when Nigel Farage, the leader of the anti-Europe UK Independence Party, lost his bid for a seat in Parliament.
Farage would have been a troublesome presence for Cameron from a perch on the opposition benches and would have delighted in poking at Conservative divisions over Europe.
Despite winning 3 million votes, in a barometer of Euro skepticism, UKIP only ended up with one of Parliament's 650 seats.
The United States in particular has made no secret of its desire to see its long-time "special relationship" ally remain a united nation and inside the European Union, believing such a position best preserves its strength and international influence.
"The biggest issue really is Europe, and what my American friends and former colleagues tell me is that there is great concern in Washington about a Britain which is leaving Europe, which may be putting that vital part of our destiny in doubt," Sir Nigel Sheinwald, former UK ambassador to the United States, told CNN.
In an interview with CNN's Christiane Amanpour ahead of the election, former President Bill Clinton said: "I would rather have them fighting within a United Kingdom, I would rather have them fighting within an EU."
He explained, "The rest of Europe needs the financial strength of the UK and the economic growth."
But many analysts believe that European leaders are unlikely to offer the kind of concessions on the free movement of people, budgets and other issues that Cameron is seeking to satisfy the British public and maintain their support for the EU - even given his enhanced political authority.
There has also been deep concern in the United States about Britain's global posture in recent years.
Austerity policies pursued by Cameron's government have meant curtailing some of Britain's military capabilities, and there are fears that London's defense spending could fall below the 2% of GDP threshold required of NATO members.
Washington has turned to another ally, France, to partner on military operations against Al-Qaeda and other groups in Africa. France and Germany have also been the White House's main points of contact on the showdown with Russia over Ukraine.
There is a clear recognition in Washington that German Chancellor Angela Merkel rather than Cameron is the most important leader in Europe.
And it did not go unnoticed in the Obama administration that the British parliament's vote against military action in Syria in 2013 helped derail Obama's own push for action against the Assad regime's use of chemical weapons.
"The UK has fallen off the world stage," one Obama administration official commented privately recently.
Cameron's other big problem is Scotland.
"The Scottish lion has roared," said veteran Scottish National Party power broker Alex Salmond, who is returning to Westminster after his party's landslide and eradication of the Labour Party in its one-time stronghold, to fight Scotland's corner in the UK parliament.
SNP leader Nicola Sturgeon repeatedly said that the election, which saw her party pick up 56 of Scotland's 59 seats was not about independence following the referendum vote against breaking away from the rest of the U.K. last year.
That strategy helped Sturgeon pick up support even from people who voted "no" in the referendum.
But the size of the SNP victory -- and that fact that the election also ushers in five more years of Conservative rule in London will inevitably -- fuel more talk of independence votes.
If Sturgeon's party repeats its performance in elections to Scotland's parliament in 2016, that momentum may become unstoppable.
Nicola Sturgeon's political challenges
Sturgeon, who emerged as an undisputed star of the election campaign, also faces political challenges. For starters, she must ensure that she does not alienate people who turned to the SNP in the general election who may be put off by a swift pivot towards another referendum.
While the SNP will have symbolic power thanks to the size of its Scottish landslide on Thursday, it will still be confined to the opposition benches in the UK parliament and will have no real power to change Scotland's relations with the national government.
Its main focus now will be in ensuring that the UK government lives up to promises of further devolution of powers to Scotland that were made in the aftermath of last year's referendum.
Cameron quickly moved to make clear he understood the implications of the triumph by SNP.
"I want to bring our country together, our United Kingdom, together, not least by implementing as fast as we can the devolution that we rightly promised and came together with other parties to agree both for Wales and for Scotland," he said.
But the sheer incongruence of the Conservatives, who are reviled in much of Scotland, and the SNP, which demonized the Tories on the campaign trail, working together will be an enormous test of the skills of both Cameron and Sturgeon.
So, as Cameron settles back into Number 10 Downing Street, he knows that the years ahead of him are deeply consequential. His task is to save the ancient union and to navigate a treacherous political relationship with Europe. How he responds will also dictate whether Britain remains America's most valued foreign ally.