The Conservative Party has long been known as the natural party of government in the UK, with members doing what it takes to win elections. But one issue has become more important for many of them than getting in to Number 10 Downing Street -– the European Union.

It all started off so well

The Conservatives' most famous leader, Winston Churchill, was an early enthusiast for the idea of European integration. He delivered a speech in Zurich in 1946, the year after the Second World War calling for "a kind of United States of Europe". It is often quoted by pro-Europeans.

But he had an important caveat for the UK's position: “We are in Europe but not of it. We are interested and associated, but not absorbed.”

The UK played no part in the forerunner of the European Union although the then Conservative leader Harold Macmillan in 1961 belatedly applied for the UK to join, only for the French President De Gaulle to say no.

It was not until there was another Conservative Prime Minister, Edward Heath, that in 1973 the UK joined what was then the European Economic Community, or Common Market. It was a move which was seen as pro-business – joining a group of trading partners at a time of economic strife in the UK. At that time it was the left-of-centre Labour Party which opposed membership.

Two years later, with Labour back in power, the UK held a referendum on whether or not to stick with membership. Labour was the party with the biggest splits over the issue, with Conservatives, including the then future Conservative prime minister Margaret Thatcher, enthusiastic backers of the UK being part of what was seen as a trading bloc rather than a forerunner of a super-state.

The Bruges speech and the splits begin

The 1980s saw Margaret Thatcher's Conservative government dominant in the UK. But there was also a switch in attitude from both major UK parties towards Europe as the evolving union moved beyond being a single market for trade, towards wider, closer, integration.

A key month was September 1988 when the then European Commission President Jacques Delors gave a speech to the UK's trade union conference setting out the "social dimension" to Europe.

Mrs Thatcher, who had curbed trade union power in the UK and slashed government spending, responded by travelling to Bruges in Belgium to deliver a direct riposte: "We have not successfully rolled back the frontiers of the state in Britain only to see them reimposed at a European level, with a European superstate exercising a new dominance from Brussels."

The party has basically been publicly split ever since that speech. It led to the formation of the Bruges Group of Conservative Eurosceptics – but it also convinced those who were enthusiastic about the UK's role in Europe, that they needed to act. Just over 18 months later the convincing winner of three general elections was forced to quit as prime minister by her own MPs, with her growing scepticism over Europe partly to blame.

At this stage the Conservative Eurosceptics were opposed to any transfer of powers from national to European Union levels, rather than campaigning for the UK to leave the EU.

New man, same problem

John Major succeeded Mrs Thatcher as Conservative leader and prime minister, and managed to scrape victory in the 1992 general election, although his majority in parliament was just 21, so only 11 rebels were needed on any issue for him to face defeat.

Unfortunately for Major, he had to try and get the UK Parliament to back the Maastricht Treaty, which created the modern-day European Union, adding the new areas of justice, home affairs and a common foreign and security policy to the existing European Community.

It also gave free movement and European citizenship to everyone living in the 12 member countries and paved the way for Economic and Monetary Union and the adoption of the single European currency.

Major was not able to win key votes without the small but determined band of Eurosceptic Tory rebels. He promised a referendum before the UK would ever consider replacing the pound with the euro and got an opt-out from the part of the treaty which covered issues such as workers' rights and health and safety. He hailed the creation of a single market and free movement of people within the EU as good for business. 

But the few Eurosceptic MPs in his party who saw Maastricht as a step towards a 'United States of Europe' embarked on a parliamentary guerilla war that saw a battered Major's room for manouevre on any policy restricted. His standing in opinion polls fell, and he was famously described as "in office, but not in power" as his party's splits continued up to and past heavy defeat and loss of office in the 1997 general election.

The wilderness years

After Major resigned, William Hague, seen as a Eurosceptic Thatcherite, beat pro-European Ken Clarke by 92-70 MPs' votes to become party leader. That result showed the party's MPs were now majority Eurosceptic. Hague made Save the Pound the key message of his 2001 election campaign. The Conservatives lost heavily again and Hague resigned the following day.

His successor was Iain Duncan Smith, an arch-Eurosceptic and one of the leaders of the Maastricht rebels. He was the first leader to be elected by party members rather than MPs. For a second time Clarke, seen to be the more likely to lead the party to general election success, lost out to a rival whose views on Europe chimed with the electorate's.

Duncan Smith didn't last long as Conservative MPs, watching their party's ratings fall further, faced a third election hammering. The veteran Michael Howard, with Eurosceptic credibility, stepped in as a caretaker but failed to boost their appeal enough and they lost a third successive election.

A third leadership election in eight years followed. Candidates, as always, were defined by their stance on Europe above everything else, including whether they might be an election winner. And by now, with incoming MPs elected by local parties whose members tended to be Eurosceptic, anyone with a positive vision of the UK's EU relations had no chance.

Ken Clarke was first to be eliminated by Tory MPs this time. David Cameron started out as a long-shot modernizer, but he saw his fortunes transformed after a Eurosceptic-pleasing pledge to take the Conservatives out of the centre-right grouping in the European Parliament.

Cameron seeks to move on from Europe

With the party keen on avoiding a fourth successive defeat the newly elected leader Cameron said they needed to "stop banging on about Europe" and focus on the issues most voters cared about. While Conservative MPs and members might see the EU as the key issue, surveys of all UK voters showed that Europe was only the 11^th biggest issue.

Cameron's strategy was to do just enough to keep his parties' Eurosceptics in line, while spending most of his time avoiding Europe as an issue. When he did talk about Europe it was to attack the Labour government for being too pro-EU, and to do things like promise a referendum before any new powers could be transferred from the UK to the EU.

Cameron's strategy helped him win enough seats for the Conservatives to form a coalition government.

But, as with John Major two decades earlier, Cameron's slim majority meant the growing band of hardline Eurosceptics had disproportionate power and influence. And they were demanding more now than curbing progess towards a European superstate.

Europe: The final countdown

The warning shot was fired by Cameron's own MPs in 2011 when 81 of them – including 49 freshly elected ones – voted in favour of a motion calling for a referendum to be held on whether to leave the EU. It was one of the biggest rebellions any Conservative prime minister had seen and it showed that the question of the UK actually leaving the EU was no longer a fringe one "for fruitcakes and loonies" as Cameron had once implied. 

The rebels combined the following year with Labour to inflict another defeat on an amendment about the EU's budget.

The rise of the UK Independence Party, with their simple message that the UK should leave the EU, meant that Cameron feared defections from his MPs and that UKIP – who had stellar European election results – could win enough votes to stop the Conservatives winning the 2015 election.

Cameron's solution in 2013 was to promise to hold a referendum if the Conservatives won the next election - on whether the UK should stay in the European Union. 

Essentially it was promised as a way of keeping control of his party and ensuring that Eurosceptics had a good reason to choose the Conservatives ahead of UKIP.

Cameron and his advisors now insist that the party's division over Europe was always going to lead to a referendum and the issue needed to be tackled proactively - but critics say the seismic vote was promised by a PM putting his party before country.

Either way, the pledge helped Cameron win a surprise majority at the 2015 election. The only question then was when to hold the referendum.

Referendum formalizes the splits

Cameron plumped for spring 2016 for the referendum, and hoped to maintain party unity by allowing people to choose which side to campaign on. Officially the party was neutral but the leadership campaigned for Remain, although six of Cameron's cabinet were at the head of the Leave campaign.

They also, crucially, got the backing of the populist former mayor of London Boris Johnson.

The party which had been becoming increasingly Eurosceptic, was now revealed to have a big chunk of MPs who were not just Eurosceptic but actually wanted the UK out of the EU. Among Conservative MPs 138 backed Leave, compared with 185 who backed Remain.

Since the Leave side won, members of the Remain side have put a big chunk of the blame on Cameron's desire not to launch personal attacks on Conservative Leave campaigners, because he expected to have to work with them after the referendum was won.

But the day the result came in a defeated Cameron knew he had no option but to quit. Europe had now claimed the jobs of three successive Conservative prime ministers, in Thatcher, Major and now Cameron.

Brexit means... Europe is the only issue

Anyone who hoped that the referendum result would lead to a quick Brexit and the Conservatives being able to forget Europe will have been sorely disappointed by events of the past three years.

Thanks to infighting amongst the Leave candidates the party ended up with Theresa May, who had backed Remain in the referendum, becoming party leader unchallenged. She pledged to "make a success of Brexit".

May inherited only a small majority from Cameron, and with a huge opinion poll lead over Labour, she called a general election in 2017, aiming to get a big majority for her Brexit stance, so she would not be reliant on the support of the hardline 'no-deal' Brexiteers to win votes in Parliament.

As it turned out, she ran a poor campaign and lost her majority, meaning that she needed the support of Northern Ireland's DUP and MPs at both extremes of the Brexit argument to win any vote.

She tried to square the circle by striking a deal with the EU which tackled the main Leave campaign issues while maintaining close trading and security links and putting off a decision on what sort of future relationship the UK and the EU would have.

Two heavy parliamentary defeats prompted several tweaks and it eventually won over prominent Leave campaigners such as Boris Johnson. But it was not a clean enough break for the 30 or so hardest-line Brexiteers among her MPs and was defeated.

After three and a bit years as prime minister she accepted defeat and resigned. The fourth Conservative PM to be brought down by the issue of Europe.

Who are the rebels now?

There is now a new man at the helm – Boris Johnson, the public face of the Leave campaign in the 2016 referendum. But the maths remains the same – with a majority of MPs against every possible Brexit option.

While he has the backing, for the moment at least, of most of his overwhelmingly Eurosceptic party, he suffered defeat over whether to have the option of a no-deal Brexit when 21 MPs from the other end of the party rebelled.

Johnson then raised the stakes, kicking them out of the parliamentary party and saying they would not be allowed to stand at the next election.

These MPs – which include the longest serving MP, Ken Clarke, as well as Philip Hammond, who was chancellor until July – are not accepting the expulsion order. Among many other complaints, the move prompted claims of hypocrisy given the number of rebellions by the one-time hardline Eurosceptics now at the heart of the Conservative government.

A handful of other pro-EU Conservatives have already left the parliamentary party in protest at Johnson's Brexit policy and as things head towards the current Brexit date of October 31, it seems that, barring a late deal being struck by the UK and the EU that can win over MPs, the issue of Europe is dividing the Conservative Party as never before. The question is whether, even after Brexit, the party can ever come together again.