In 1938 Heusaff joined the pro-independence Parti National Breton (PNB). At around the same time he also became a member of Kadervenn, a nucleus of armed fighters organized by Célestin Lainé together with Alan Le Louarn. The latter aimed to make Kadervenn the embryo of a Breton army and recruited many former members of the clandestine organization Gwenn ha du, which had been responsible since 1932 for carrying out a number of armed political attacks. He consequently participated in the secret manoeuvres held at Lanvaux, in the south of Brittany in 1938.
In an interview given to Ronan Caerléon in 1970, Heusaff justified his collaborationism:
From 1938 onwards I shared the conviction that Brittany could never regain her freedom "by consent"; the French state would use all its strength to prevent that ever happening. I agreed that we should seek external support, wherever it came from, because we were too weak to attain our aims alone. Why should we not do what all free countries do when their freedom is threatened: seek alliances? By doing so we were affirming that we were already free. To be free, one must accept that there is a price to be paid: breaking with one’s family (who fear for one’s future), with neighbours (who do not wish to see one breaking ranks), with the majority of one’s compatriots who have not yet seen that they are being treated worse than slaves and wish to be doubly French. There was a strong possibility that the road would end in prison or death. I therefore committed myself to armed insurrection under Lainé, the nationalist leader whose actions were most in tune with the idea of the Breton nation.
Once it became clear to me that the Catholic Church in Brittany was lowering itself to assisting the French state in its policy of exterminating the Breton language and opposing the nationalist movement I rejected its authority and, in the end, its dogmas, in order to seek, in the best finest works of the Celtic culture of Ireland, a philosophical base more in accordance with what I wanted for Brittany in particular.
I am not greatly bothered by accusations of the kind appearing in French newspaper articles, and even in certain works written by Bretons, that ‘the Bezen Perrot committed as many crimes as the Milice’. Were all the fighters on one side saints and, on the other, brutes? As far as I am concerned, the action I saw with the Bezen Perrot was no different from that engaged in by fighters in other wars, and it is a truism that war is not a game. I was wounded seriously enough to warrant more than four months in hospital, and that was in an open fight in the course of which "Larnikol" from Plobannalec and "Lezet" from Saint-Malo gave their lives for Brittany.
In 1950 Heusaff resumed his studies at University College Dublin, subsequently finding employment with the Irish National Meteorological Service. In 1953 he married Bríd Ní Dhochartaigh, from Donegal. They were to have six children.
Heusaff returned to Brittany at least once, following the death of Célestin Lainé in 1980, when he took part in the ceremony of scattering Lainé’s ashes on the battlefield at Saint Aubin du Cormier.
In August 1995, the periodical Al Liamm published an article by Heusaff in which he defended the holocaust denier Robert Faurisson, asserting that his research was "meticulous and honest". He contributed regularly to Bremañ, in particular in 1998. Until his death in November 1999, Heusaff fought for the establishment of a federation of Celtic nations within the European Union.
The first all-Breton dictionary, An Here, also cited him as a major contributor, which provoked protests, given Heusaff’s political past. His name was not included in the (augmented) second edition as he did not participate in it.