Welsh nationalism was ignited in 1936 when the UK Government settled on establishing an RAF training camp and aerodrome at Penyberth on the Llŷn Peninsula in Gwynedd. The events surrounding the protest, known as Tân yn Llŷn ("Fire in Llŷn"), helped define Plaid Genedlaethol Cymru. The UK Government settled on Llŷn as the site for its training camp after similar proposed sites in Northumberland and Dorset met with protests.
On 8 September 1936, the building was set on fire and in the investigations which followed Saunders Lewis, Lewis Valentine, and D. J. Williams claimed responsibility. They were tried at Caernarfon, where the jury failed to agree on a verdict. The case was then sent to be retried at the Old Bailey in London, where the "Three" were convicted, and sentenced to nine months imprisonment. On their release from Wormwood Scrubs, they were greeted as heroes by 15,000 Welsh people at a pavilion in Caernarfon.
In 1936, in the midst of the turmoil of Tân yn Llŷn, Lewis praised Adolf Hitler when he said "At once he fulfilled his promise — a promise which was greatly mocked by the London papers months before that — to completely abolish the financial strength of the Jews in the economic life of Germany." However, in the 1930s other British politicians also made favourable comments about fascist leaders.