‘If graffiti changed anything, it would be illegal’. Daubed onto a wall in London’s busy, trendy Fitzrovia district by Britain’s most celebrated street artist, one of Banksy’s most famous slogans is of course intended to be ironic. Graffiti really can change things – that’s why it’s illegal.

The  village of Llanrhystud is about as far as you can get from Fitzrovia. But this remote, rural site may be the best place to prove the enigmatic artist’s tortured argument.

Ten miles south of Aberystwyth on a quiet section of the A487 coast road stand the remains of a stone cottage called Troed-y-Rhiw. Its walls would have crumbled by now were it not for a chance decision by a fiery young poet over half a century ago.

It was here that Meic Stephens and his friend Rodric Evans decided to stop their car and paint the now famous rallying cry ‘Cofiwch Dryweryn’ into the very history of Wales.

Translated into English as ‘Remember Tryweryn’, the simple two word message was a reference to the damming of the river of that name, north west of Bala in Gwynedd, and the consequent drowning of Capel Celyn, one of the last Welsh-only speaking communities in the country.

In 1960, a bill sponsored by Liverpool City Council was brought before Parliament in Westminster to develop a water reservoir in the valley.

By obtaining an Act of Parliament, Liverpool could bypass local authorities and a proper planning inquiry. And although all but one of Wales’ MPs voted against the bill, the act was passed anyway – easily carried on the dominance of English votes in the House of Commons.

In 1965, the village of Capel Celyn – twelve houses and farms, a post office, a school and a chapel with a cemetery – was submerged, and 48 people lost their homes.

The National Wales: The drowned village of Capel Celyn in Gwynedd emerging from the Tryweryn resevoir after a dry summer.The drowned village of Capel Celyn in Gwynedd emerging from the Tryweryn resevoir after a dry summer.

This outrage was to change the Welsh political landscape forever. Until then Wales’ nationalist movement had been characterised as a fringe or even ‘extremist’ endeavour.

Now it had a cause célèbre the whole nation could understand. Wales – its people, its communities, its heritage, its language – had been maltreated, steamrollered by an uncaring British state.

Meic Stephens painted the ‘Cofiwch Dryweryn’ mural in protest, and then in 1966 Gwynfor Evans won Plaid Cymru’s first ever parliamentary seat in the Carmarthen by-election.

Two years later, R. S. Thomas published ‘Reservoirs’, his brutal poetic assessment: the drowned valleys at Tryweryn and later Clywedog – which provides water for the English West Midlands – were ‘the subconscious of a people’, symbols of ‘a dead Nation’ and ‘the English’ were ‘elbowing our language into the grave’.

It was a turning point in Wales’ relationship with the British state that is even clearer in hindsight. And Thomas was by no means the last artist to draw on the reservoirs for inspiration.

Wales’ cultural and political renaissance in the decades since the haunting nadir of Capel Celyn can be traced through the work of artists as diverse as Enya and the Manic Street Preachers, indie band Los Campesinos!, English composer Michael Stimpson and electronic musician Bibio - all of whom have produced tracks remembering Tryweryn.

In 1997 visual artist Tim Davies created a floor installation – five thousand cast wax nails – inspired by the discovery of a rusty nail on the dried up bed of the reservoir.

In film, the 2010 drama Patagonia by Welsh director Marc Evans, and the 2016 Welsh-language film Yr Ymadawiad (The Passing) both poignantly explore aspects of the story.

Even the recent Netflix hit series The Crown makes mention of Capel Celyn in an episode entitled ‘Tywysog Cymru’, in which a young Prince Charles stays at Aberystwyth University to learn Welsh.

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Earlier this year, broadcaster Huw Stephens (the late Meic Stephens’ son) presented a major BBC television series on The Story of Welsh Art.

Fittingly, the tale begins with the original Welsh wall writing – on the rocks at a burial chamber on Anglesey – before exploring how Celtic crosses, landscape paintings and Bedwyr Williams’ foil blankets have all contributed to the development of a specifically Welsh art history. But his dad’s graffito made it into the documentary too – and rightly so.

You can now get ‘Cofiwch Dryweryn’ on t-shirts, mugs and keyrings – even bedspreads!

With the recent growth of Yes Cymru and a resurgence of interest in the mural after repeated incidents of vandalism during the last decade, the slogan has crossed from Welsh nationalist history into the mainstream, ensuring that the mural – and the story behind it – will indeed never be forgotten.

It’s no exaggeration to say that if it had not been for Capel Celyn, there may not have been a Senedd election at the weekend. The protests galvanised by the flooding of the Tryweryn Valley led directly to a much stronger movement for a greater degree of self-governance in Wales.

But despite the eventual success of the devolution referendum in 1999 and over twenty years of what was the Assembly, now Senedd Cymru – the Welsh Parliament – the root cause of the tragic mistake made at Westminster over Capel Celyn persists.

"We often struggle to make our voices heard in Wales," Plaid Cymru leader Adam Price remarked to BBC presenter Andrew Marr in recent days. And it would be hard – even for the staunchest of unionists – to disagree with this complaint.

‘Cofiwch Dryweryn’ is a cry in the wilderness that will echo forever in Wales' history. Unlike the unfortunate valley itself, the iconic words on that wall in Llanrhystud should never be erased.

Graffiti really can change things, and remembering what Tryweryn was is remembering what Wales might yet become.