Twenty years since the creation of the Scottish Parliament, the National Assembly for Wales and the Northern Ireland Assembly, it is clear that devolution is a source of strength for the UK – not a sign of its weakness.
It is the form of government best suited to our geography, our history and our future.
Stormont, Holyrood and Cardiff Bay are democratic expressions of the multiple identities that define the Union.
And devolved legislatures working alongside a United Kingdom Parliament elected by every citizen of the Union, containing representatives of every community in the Union, means the best of both worlds.
The benefit of more responsive and representative government for Scotland, Wales and Northern Ireland, without sacrificing the strength and security that pooling and sharing risks and rewards provides.
Northern Ireland had a devolved Parliament from its inception up until the beginning of the Troubles, after which it always remained UK Government policy to see devolved government there restored.
And when devolved government in Northern Ireland was finally restored, it was after an historic agreement to end a long and painful period of conflict.
The devolution settlement brought about by the Belfast Agreement was an example for the world in uniting people behind a shared future.
And as the parties in Northern Ireland continue in talks to restore that devolution, the hope and history surrounding it should be a powerful reminder of the imperative of not letting that progress slip away.
In Great Britain, successive Governments of both parties pursued policies of administrative devolution – the creation of separate Whitehall departments for Scotland and Wales, led by cabinet Ministers – but resisted calls for legislative devolution.
There were objections to it from across the political spectrum.
Many on the left feared it would weaken the ability of a socialist government to effect economic and social change.
And, despite first making a Scottish Assembly our policy as early as 1968, many Conservatives worried that it might loosen the bonds that tie us together.
Both left and right are now fully united behind devolution.
A Labour Government created the devolved institutions in Scotland and Wales.
And Conservatives have embraced them.
Successive Conservative and Conservative-led governments since 2010 have strengthened the devolution settlements.
Holyrood has new powers over tax, welfare and more.
Cardiff Bay has tax powers and law-making powers.
In Northern Ireland, we have legislated to enable Stormont to take on powers over corporation tax.
Today, the only threat to devolution comes from those parties who want to end it by breaking up the United Kingdom.
For those of us who believe in the Union, devolution is the accepted and permanent constitutional expression of the unique multinational character of our Union.
It was ironic that the UK Government's sincere efforts to ensure that Brexit had no unintended consequences for the UK's internal market was dismissed as a 'power-grab' by the SNP.
A UK Government which had enthusiastically launched and implemented the Smith and Silk Commissions – transferring sweeping powers over tax and welfare, stood accused of using Brexit as the cloak behind which to claw-back powers over food labelling and fertiliser regulations.
On one level, the allegation is simply absurd.
On another, it highlights a challenge which faces the UK Government as it seeks to act in the best interests of the whole UK.
Whereas the UK Government is invested in the success of devolution, it would suit the political aspirations of the present Scottish Government for devolution to fail, or to be seen to fail.
The criticisms of the present First Minister about how our two governments interact need to be viewed in that context.
It is telling that during the discussions over legislative consent for the EU Withdrawal Bill, after intense discussions and give and take on both sides, the Welsh Government was willing to making a compromise, whereas the Scottish Government was not.
Over the last three years, I've learned that while other parties can be relied on to work with the UK Government in good faith to make devolution a success, an SNP Scottish Government will only ever seek to further the agenda of separation.
That, I am afraid, is simply a fact of political life in the UK at the moment.
That fact puts an additional responsibility on the UK Government.
If we do not do all we can to realise the full benefits of the Union – no one else will.
If we do not use every policy lever within our reach to strengthen that Union – no one else will.
And if we do not make realising the full benefits of being a United Kingdom of four proud nations and one united people our priority now, then in the future it may be too late.
The answer does not lie in further constitutional change – or in reimagining what the Union is or should be.
Well-intentioned suggestions that we should, for example, seek to agree a new Act of Union for the 21st century ignore the political reality.
With good will on all sides such a thing might be possible – but we do not have good will on all sides.
Many of those who advocate a federal UK are equally well-intentioned, but I believe are also in the wrong track.
England makes up over 80% of the UK population.
There is no example of a federal state anywhere in the world where one of the units of the federation is so large.
The UK simply does not lend itself to federation as a sustainable constitutional model.
The only way it could realistically be achieved would be by breaking England up into artificial regional units – something I would never support and for which I detect no appetite.
Of course, that is not to say that there is no appetite for devolution in England.
The UK Government has passed considerable power down to the great cities and metro-areas of England.
From Greater Manchester and the West Midlands to the Tees Valley and Bristol, there is now a new cast of powerful, directly-elected local voices speaking up for their areas – voices which great cities like Glasgow and Cardiff lack.
So the answer to strengthening the Union does not lie in schemes of sweeping constitutional change, but in making better and more creative use of the powers and potential of the constitutional settlement we have.
The City and Growth Deals which the UK Government has pioneered across the United Kingdom – from Aberdeen to Swansea, Derry/Londonderry to the Borderlands – are examples of that creative thinking.
Working with the devolved administrations and local authorities as partners, they provide a vehicle for UK-wide engagement – each layer of government working together to drive better outcome for citizens.
The leadership election in my own party has encouraged a raft of suggestions for how the UK Government can play a more constructive role in realising the full benefits of the Union for all its people.
It has been a striking change at Westminster since the 2017 election that we now have a range of passionate and articulate Scottish voices across the House of Commons making constructive arguments about how to make the UK a better place.
But we will need to keep up this debate and for it to be informed by creative thinking and new ideas.
Tweaking the constitution is not the answer.
The UK Government already invests significant amounts across the nations of the UK, and the Barnett Formula rightly delivers higher public spending per head in Wales, Northern Ireland and Scotland – so spending alone is not the answer, either.
Instead, we need to look afresh at how we use the levers and the resources that are to hand through a Unionist lens.
We need to work more cleverly, more creatively and more coherently as a UK Government fully committed to a modern, 21st century Union in the context of a stable and permanent devolution settlement to strengthen the glue that holds our Union together.
There have been several reviews into how devolution works.
But we have never thought deeply about how we make the Union work – how we ensure that as we fully respect devolution, we do not forget the UK Government's fundamental duty to be a government for the whole United Kingdom of Great Britain and Northern Ireland.
And that is why I have asked Andrew Dunlop to lead an independent review into the structures of the UK Government to ensure that they are set up to realise fully all the benefits of being a United Kingdom.
Lord Dunlop has a wealth of experience from his time in Government as an advisor and Minister and I look forward to reading his report.
Of course, it will be for my successor to respond to his recommendations, and I am delighted that both candidates are supportive of the review.
I am confident that whoever succeeds me in 10 Downing Street will make the Union a priority.
He will build on the work of a UK Government that has made strengthening the Union an explicit priority.
The job of Prime Minister of the United Kingdom of Great Britain and Northern Ireland brings with it privileges and responsibilities which you only really feel once the black door closes behind you.
One of the first and greatest is the duty you owe to strengthen the Union.
To govern with the popular support on which that Union is based.
To respect the identities of every citizen of the UK – Scottish or Welsh, Northern Irish or English, British or Irish.
And to ensure that we go on facing the future together, overcoming obstacles together, and achieving more together than we ever could apart – as a Union of nations and people.