Abraham Lincoln’s Emancipation Proclamation of 1863 is one of the noblest statements ever delivered, and forcing the abolition bill through a reluctant Congress, as Steven Spielberg’s masterful Oscarnominated film attests, was a monumental achievement. But Lincoln’s principal contribution to American history was to save the Union, as those from the Southern states are quick to tell you. In the former Confederacy, the civil war is still called “the War Between the States”.

Lincoln confided his thoughts about secession and slavery in a letter of 1862. “If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that,” he wrote. “What I do about slavery, and the coloured race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union.”

His proclamation did not, in fact, free the slaves in the North, nor was he in a position to free slaves in the Confederate South, but, under his powers as commander-in-chief in wartime, he issued an executive order that freed all slaves in the Southern states as soon as they were occupied by the Union army.

It may at first seem a little far-fetched, but there are poignant similarities between the conundrum that Lincoln encountered 150 years ago and the dilemma David Cameron faces today. They are both confronted with threats to the very existence of the nations they govern. While Lincoln was obliged to respond to a fait accompli, a group of slave states that had decided before his election to wrest themselves from the Union, by force of arms if necessary, Cameron finds himself under siege on all sides. But while Lincoln was presented with the simple option of whether to take up arms to defend the Union or watch as his country split in two, Cam eron has no such easy choice.

In Scotland, the Scottish National Party has finally achieved what it has been dreaming of for 80 years. It has a mandate to demand from Westminster a referendum on whether, after three centuries united with England and Wales, Scotland should become a free nation again. The Union came about as a result of the Union of the Crowns, when the Scottish king James VI, son of Mary, Queen of Scots, acceded to the throne of England following the death of the childless Elizabeth I in 1603. It took a full century before the English and Scottish parliaments combined in the Acts of Union of 1707. Lincoln was obliged to defend a union barely 90 years old; Cameron must protect a union that has lasted more than 300 years.

In Ireland, Cameron presides over the latest skirmish in a bloody struggle that has lasted much longer. The colonisation of Ireland was messy and brutal from the start, and the independence wrested from Britain in 1922 left the northern, overwhelmingly Protestant and unionist part of the island in British hands. A border had to be drawn somewhere, leaving many who would prefer to live in the republic stranded in a British province. The continuing troubles offer a challenge to Cameron to find a permanent peace. No less than in Scotland, British sovereignty and British lives are severely at risk.

Then there is the European Union. Those with a sense of history will remember that joining Europe was always predominantly a Conservative project. It was Harold Macmillan, with Edward Heath at his side, who first flirted with the continentals in 1961 and had his overture rudely rebuffed by Charles de Gaulle’s “Non!”. Heath the eternal bachelor then made it his life’s mission to make a marriage with the Europeans and the lasting legacy of his otherwise awkward, chilly and ultimately tragic premiership was British entry into the European Economic Community in 1973. As Cameron must be all too aware, the principled Heath condemned the referendum that Harold Wilson called on European membership two years later as a shabby gimmick, designed to appease internal Labour divisions over the Common Market.

Since the moment when Heath’s successor Margaret Thatcher – who had campaigned in favour of remaining in Europe in 1975 – began arguing, as prime minister after 1979, against closer European union, the Conservatives have been profoundly and openly divided on the matter. The rupture over Europe, even more than Thatcher’s unpopular poll tax, led to her defenestration by cabinet colleagues in 1990. John Major’s leadership of the Tories was blighted by the question of Europe; and the election of three Eurosceptic leaders in a row – William Hague, Iain Duncan Smith and Michael Howard – did not settle the matter.

Cameron’s inheritance is a party facing both ways on Europe, and his inability to reconcile the opposing forces has given rise to a challenge for the affections of his patriotic electoral base from the anti-European Ukip. Although Ukip’s leader, Nigel Farage, along with every other Ukip candidate, failed to win a Commons seat in 2010 (Farage was beaten by a candidate dressed as a dolphin), his party stole enough votes from the Conservatives to deprive Cameron of a parliamentary majority.

When he dreamed of leading his party, Cameron could never have imagined that Britain’s existence would be subject to a three-pronged attack. But he finds himself in the same position as today’s Republican leadership in America, under assault from angry rank and file who feel they are being ignored and betrayed by their leaders. The Republicans, once the proud “party of Lincoln”, have evolved into a testy vehicle for insurgent mavericks and malcontents.

To add insult to indignity, the “Grand Old Party”, which once bravely saved the Union, is the home of a new secessionist movement. Having failed to devolve substantial powers from the federal government to the states, many are demanding independence. At present, eight states, all from the defeated Confederacy, have petitioned the White House to be allowed to secede: Texas, Louisiana, Florida, Georgia, Tennessee, North Carolina, Alabama and South Carolina. The muddled, ahistorical thinking behind the treacherous talk is evident in the argument proffered by the libertarian Ron Paul: “It’s very American to talk about secession. That’s how we came into being.”

On a personal level, there are as few similarities between Cameron and Lincoln as between Jacob and Esau. Lincoln was brought up in a sparsely furnished log cabin and, much to his ignorant father’s despair, taught himself to read and write, eventually emerging as a jobbing country lawyer in Illinois. Cameron, as we know, was the son of high privilege. Everyone who met Lincoln commented on his rough looks and his even rougher clothes. Cameron’s smooth, unlined face betrays an easy, affluent, well-fed life.

Both men, however, could be described more accurately as Whigs than Conservatives, in their commitment to parliament or Congress over absolute powers held by the monarch or president. Indeed, Lincoln was an old American Whig before he joined the Republicans over the issue of abolition. Allied to their commitment to rewarding individual effort, irrespective of background, is a strong, Protestant sense that their good fortune entails paying something back. Despite his comfortable circumstances, Cameron has argued that “it’s where you are going to, not where you have come from, that matters”. In a decisive break from the philosophy of heroic individualism that inspired Thatcher, he believes “there is such a thing as society”.

As well as soaring ambition, the two men share other similarities. Both are most eloquent when they do not refer to notes. Although stiff and wooden at first, Lincoln’s speeches gathered pace and by the peroration he would be ripping off his necktie, loosening his starched collar and throwing his arms around like a deranged windmill. “His pronunciation is bad, his manners uncouth and his general appearance anything but prepossessing,” is how one eyewitness described his platform presence.

Cameron’s delivery is calm, ordered and deliberate. His speech to the Tory party conference in 2005, delivered without notes, may not have been as powerful and inspirational as the 268 words of Lincoln’s Gettysburg Address of 1863, which would be a tall order for anyone except, perhaps, Winston Churchill. But the performance at Blackpool, in its carefully pitched content tailored to the party faithful and the confidence of its delivery, ensured his election as leader.

Lincoln took into his administration the big beasts of the Republican Party whom he had beaten to the Republican nomination: William H Seward, Salmon P Chase and Edward Bates. And Cameron, too, assembled a team of former rivals. To become Tory leader, he saw off David Davis, Liam Fox and Kenneth Clarke, all of whom he invited into his shadow cabinet. Like Lincoln, Cameron leads his disparate colleagues with the minimum of friction. But there the favourable comparisons between the two leaders start to run out.

Lincoln was always a man of principle rather than pragmatism. He could be rash, failing to hold his tongue in the presence of those he knew disagreed with him, and found it difficult to compromise even when it was in his best interest to do so. Nowhere was this more obvious and powerful than when he spelled out, years before running for the White House, what he felt about race.

He declared that when the Founding Fathers wrote, “We hold these truths to be selfevident: that all men are created equal,” they meant “the whole great family of man” and not merely those with white faces. Lincoln said the founders knew enough about human nature to imagine that, “in the distant future”, people would emerge who would “set up the doctrine that none but rich men, or none but white men, were entitled to life, liberty and the pursuit of happiness”. But he was certain that racism could never have been in the founders’ minds and he would have none of it.

In comparison to this eloquent statement of principle, just one among dozens that Lincoln crisply articulated in his short life, Cam - eron emerges as a dissembler, always alert for a way to delay taking a stand, ever ready with the smudgy phrase and the tactical retreat. Let us give him a pass on Ireland. Few have got it right and it may well be insoluble so long as a vociferous minority in Northern Ireland demands the impossible and the intractable majority insists on being British. In Scotland, however, when the SNP obtained a majority in the Scottish Parliament and claimed a mandate to call a referendum on independence, Cameron readily ignored Lincoln’s example to resist the dissolution of the Union and readily agreed to Alex Salmond’s demands.

In calling an all-or-nothing, in-out referendum on independence next year in Scotland only, David William Donald Cameron, to give him his full, Scots-derived name, failed to question the legality of one half of the nation being able to secede from the other on its own cognisance. Instead, he conceded the principle that if the referendum records a majority of Scots in favour of secession, that is enough to grant a divorce, as if England, Wales and Northern Ireland, and the Scots living in the rest of Britain, were not entitled to a say in the dissolution of the United Kingdom. “I’m not going to stand here and suggest Scotland couldn’t make a go of being on its own, if that’s what people decide,” Cameron said. “There are plenty of small, independent nation states of a similar size or even smaller. Scotland could make its way in the world alongside countries like those.”

Lincoln would never have yielded on such a fundamental principle. As he put it, “If we do not make common cause to save the good old ship of the Union on this voyage, nobody will have a chance to pilot her on another voyage.”

When Cameron conceded the principle that one part of the United Kingdom may constitutionally break from the rest, he also declared himself “ready for the fight for our country’s life”. He appears to be in favour of two incompatible principles, the right of Britain to remain a nation and the right of Scotland to secede. He then adopts the principle that gives Scotland the moral right to secede to inform his party’s demand that Britain be allowed to renegotiate a looser union with our European partners. What, then, is Cameron’s guiding principle when dealing with Scotland and the European Union? There is none. Both are craven acts of political expedience. His promise of a referendum on British membership of the EU is largely an attempt to save the Conservatives from being driven from office by Ukip.

Cameron’s answer to the Ukip threat to the renewal of his Downing Street lease is to avoid saying exactly what the relationship between Britain and the EU should be, because plainly he doesn’t know where the line should be drawn. Instead he abrogates the responsibility of a true leader and, in the hope of being re-elected, promises an in-out referendum on EU membership, so long as he is re-elected. As Lincoln asked, “What is conservatism? Is it not adherence to the old and tried against the new and untried?”

Cameron is less a conservative than a trimmer, less a Heath than a Wilson, less a That - cher than a Blair.

When Lincoln confronted the break-up of the United States, he borrowed from the Gospel according to Saint Matthew: “A house divided against itself cannot stand.” To avoid the consequences of the Conservatives’ deeply divided house, Cameron is willing to risk the dissolution of the United Kingdom and British withdrawal from the European Union. Both are too high a price to pay for trying to bridge the irrevocable schism in the Tory ranks.

Nicholas Wapshott’s most recent book is “Keynes Hayek: the Clash That Defined Modern Economics” (W W Norton, £12.99)