A native Breton speaker, he trained as a primary school teacher but in his early twenties joined the separatist Bezen Perrot militia (1943–44), for which he was sentenced to death in absentia at a court martial by the post-World War II French government, but eventually amnestied in 1967.
An aviation meteorologist, he devoted his spare time and retirement to peaceful activism, promoting the languages, culture and autonomy of the Celtic countries.
Among the honours he received for his work was the 1986 Gradam an Phiarsaigh (annual Pearse award) presented by the President of Ireland, Patrick Hillery.
In 1925 the French Minister of Education, Anatole de Monzie, made clear the Government policy: "For the linguistic unity of France, Breton must be exterminated".
Nevertheless, Heusaff had a good ear for languages and eventually joined the École Normale in Kemper (Quimper, Finistère) where he trained as a primary-school teacher.
Following the death of Charles VIII in 1498, Anne was forced to marry his cousin, Louis XII of France, to ensure the French crown's continued control of Brittany.
This caused a complicated situation in Brittany as many Bretons had spearheaded the Revolution as a means of overthrowing the centralist politics of the French monarchy.
The establishment in July of Marshal Pétain's French collaborationist government in Vichy, however, still gave it legal authority not only in the "unoccupied south" but also in northern and western France occupied by the German Wehrmacht.
Rather than help the Bretons achieve their freedom, the German Occupation allowed the French collaborationist government of Vichy to remove a large section of Brittany, the department of Loire-Atlantique, in 1941.
But others, including militant activist Célestin Lainé (later known as Neven Henaff), continued to make overtures to the Nazis, hoping for their support for an independent Brittany with ties to Germany.
(7) The war divide within Brittany as a whole deepened at the same time and members of the Maquis, the French Resistance, began to view all Breton nationalists as potential collaborators.
Perrot had been decorated for his services in World War I, but was a native speaker and leading cultural Breton nationalist, playwright and writer, involved in devising a standard orthography for the language.
[citation needed] Célestin Lainé had led an underground physical force movement, Gwenn-ha-Du (white and black, named after the Breton national flag) from 1930, had organised militant groups such as Lu Brezon, renamed Bezen Kadoudal, and now saw the opportunity to organise an open, uniformed and armed group – Bezen Perrot.
The conditions of this unit was that it would not fight outside the borders of Brittany but remain as "a protective militia" against the French Maquis's attempts to eliminate Breton activists.
The main focus of the book is an analysis of why Ireland gave asylum after the war to some Bretons and other foreign militants who had collaborated with Axis forces.
Controversies on these issues are also elucidated in Leach's article "Irish Post-War Asylum: Nazi sympathy, Pan Celticism or raisons d'etat?"
He had connections with the Deutsche Gesellschaft für keltische studien (German Society for Celtic Studies) in Berlin and had been on a research trip to Brittany in 1943.
Of those involved in the Breton movement, 38 had been killed and 9 executed by firing squad, according to contemporary sources such as the Welsh newspaper Baner Ac Amserau Cymru.
(15) On 20 May 1950, Heusaff left Marburg under the alias "Bernard Heubacher", and travelled through Belgium to England and Wales, and then to Ireland, where he enrolled in University College, Galway, to finish his degree.
After his death, Brendan McWilliams, director of the service, and a well-known Irish Times columnist, wrote:Alan spent his entire working life in aviation meteorology...
McWilliams adds: "He was a meticulous and conscientious meteorologist, and I personally remember him as always cheerful, courteous and eager to help to solve whatever operational problems might arise."
(18) The principal aims were then: (1) to foster co-operation between the national movements in the Celtic countries, particularly in efforts to obtain international recognition and to share the experiences of their struggles and exchange constructive ideas.
[citation needed] The League then launched its own quarterly journal Celtic News, initially edited by Welsh historian Dr. Ceinwen Thomas of University College, Cardiff.
[citation needed] Gwynfor Evans MP continued as president until 1972, while Vice-Presidents consisted of leaders of the main political national parties, such as Dr Yann Fouéré of the Mouvement Pour l'Organisation de la Bretagne; Dr Robert McIntyre of the Scottish National Party (and its first Member of Parliament) and Robert Dunstone, the president of the Cornish movement, Mebyon Kernow.
[citation needed] Under Heusaff, the League gave evidence to the European Commission of Human Rights in 1963 concerning the persecution of the Breton language.
[citation needed] The League also took a leading part in other campaigns, for example, monitoring submarine activity in the Irish Sea where they refused to surface, snaring fishing nets and causing the loss of boats and lives.
The League soon made the idea of Pan-Celticism a public issue and forced the academic Celtic Congress into actions such as adopting a "Charter of Cultural Rights".
Upon his retirement from the Meteorological Service in 1986, Alan and Bríd moved to Seanadh Gharráin, near Spidéal, County Galway, where he concentrated on his lexicographical work, kept up a voluminous correspondence with his Celtic contacts, and remained involved in the League.
We need to think about what contribution the Celtic peoples and their cultures can make to the development of European unity, to formulate proposals and suggestions, disseminating them, instead of adopting an aloof sort of attitude.
He also contributed to Breton language magazines in Brittany such as Hor Yezh, Galva, Breman, Gwalarn, Arvor, Ar Bed Keltiek, An Amzer and Al Liamm.