This was the core message of the Tory leader's speech as he reached out to Ulster Unionist members at their annual conference on Saturday.

In an impassioned and convincing defence of the Union, Mr Cameron pushed all the right buttons.

Perhaps crucially for a significant block of older UUP members, he made an effort to lay to rest 23 years of hurt over the loathed Thatcher-backed Anglo-Irish Agreement which drove a huge wedge between the Tories and UUP in 1985.

While he did not specifically name the accord, he said he and his party had regrets and had made mistakes, and it was clear what he was referring to.

In short, he came as close as he could to apologising for a pact, that was nothing to do with him, but once gave the Irish Republic an influence on Northern Ireland's internal affairs.

"The links between our two parties are long and intimate," said the Tory leader.

"As with any long relationship, we've had our disagreements and misunderstandings.

"I acknowledge we've all made mistakes and I regret that," he declared.

And even more explicitly, Mr Cameron went on to reference and reverse the words of former Conservative Secretary of State Peter Brooke, who in 1985 proclaimed his party had no selfish strategic or economic interest in being in the Province.

Having Northern Ireland firmly settled within the United Kingdom was "in the interests of Northern Ireland", said the Tory leader, and "in the interests of the United Kingdom".

Furthermore, he pledged, as a committed unionist and would-be Prime Minister, "It's in my own selfish interests too".

It was a pivotal moment in the speech, on what could prove to be a pivotal day for unionism and conservatism, both.

Mr Cameron added: "As Prime Minister I will always honour Britain's international obligations.

"I will continue to work closely and constructively with our nearest neighbours in the Republic of Ireland and I will always uphold the democratic wishes of people here in respect of their constitutional future.

"But I will never be neutral when it comes to expressing my support for the Union."

The Conservative boss set out to explain the reasoning behind the UUP deal which will see the parties field joint candidates at the European and Westminster elections.

For the Conservatives, he noted there could and would be no "no go areas" in UK politics.

But essentially he broke the thinking down to three areas: a deep commitment to the Union, a strong belief in democracy and giving people in Northern Ireland a real say in government, and "a great respect for the Ulster Unionist Party".

On the Union, he stressed: "I've never been a little Englander.
"I passionately believe in the Union and the future of the whole United Kingdom.

"We're better off together, England, Northern Ireland, Wales and Scotland, because we all bring our strengths to the mix."

He continued: "Together, we are the fifth largest economy in the world.

"Together, we have a seat at the top table and are listened to in a way that other countries can only dream of.

"Together, we have one of only five permanent seats of the United Nations Security Council.

"Together we are a major player in the EU, in NATO and other international organisations.

"And together, we have the British military, one of the most respected armed forces in the world."

Separately, this would not be so.

Mr Cameron's speech went on to speak of the history of the Province and the Union and the ties that bind, in particular, the way in which the nations of the UK have stood together in times of war.

On the democratic potential of the UUP pact, the Tory leader said the constitutional point was settled, and the deal will provide a chance for people in Ulster to participate at all levels and "right the way to the Cabinet table itself".

The Assembly and devolution are a good thing, he said.

But he noted, only in Downing Street could: taxes be kept as low as possible, VAT holidays be introduced to help small firms with their cash flow, the small companies tax rate be reduced to 20 per cent and the main rate to 25 per cent, and employers' national insurance rates be taken down one per cent for the smallest firms.

And, he added, only a Conservative Government led by him will look at the shared land border with the Irish Republic and address the impact that has on inward investment and Northern Ireland's economic competitiveness.

He concluded: "The semi-detached status of Northern Ireland politics needs to end.

"It's time for Northern Ireland to be brought back into the mainstream of British politics."