Animal liberation ARM first emerged in the United Kingdom (UK) in the 1980s as animal-rights activists shifted their focus away from demonstrations and more on direct action, including violence, intimidation, and the destruction of property. In 1982, letter bombs were sent to Margaret Thatcher, then British Prime Minister, signed by the Animal Rights Militia. The group was not heard of for four years afterwards, and animal liberation supporter Peter Singer remarked in his essay "The Animal Liberation Movement: Its philosophy, its achievements and its future" that the group may not really exist. 
In 1986, ARM claimed responsibility for sending letter bombs to individuals involved in vivisection, and in 1994, ARM activists set fire to stores on the Isle of Wight, causing 3 million pounds worth of damage. Barry Horne was subsequently jailed for eighteen years for the arson attacks. He died in prison in 2001 during a hunger strike. Robin Webb, who runs the Animal Liberation Press Office in the UK, narrowly avoided being charged with conspiracy.
ARM came to widespread public attention in the UK in December 1998, during one of Horne's earlier hunger strikes, which lasted 68 days. It was carried out in protest at the British governments refusal to order a commission of inquiry into animal testing, and ARM threatened to assassinate a number of individuals involved in vivisection should Horne die.
Those threatened were Colin Blakemore, later chief executive of the Medical Research Council; Clive Page of King's College London, a professor of pulmonary pharmacology and chair of the animal science group of the British Biosciences Federation; Mark Matfield of the Research Defence Society; and Christopher Brown, the owner of Hillgrove Farm in Oxfordshire, who was breeding kittens for laboratories.
Robin Webb has implied that ARM and Animal Liberation Front (ALF) activists, as well as activists from another violent group, the Justice Department, may be the same people. He has said, "If someone wishes to act as the Animal Rights Militia or the Justice Department, simply put, the third policy of the ALF, to take all reasonable precautions not to endanger life, no longer applies."
Professor Paul Wilkinson, former director of the Centre for the Study of Terrorism and Political Violence at the University of St Andrews, has said that "a cluster of small groups such as the Justice Department, the Animal Liberation Front, and the Animal Rights Militia, have crossed the threshold from extra-parliamentary protest and demonstrations to what can only be described as acts of terrorism."
ARM claimed responsibility for removing from a grave in October 2004, the body of Gladys Hammond, the mother-in-law of Christopher Hall, part-owner of Darley Oaks Farm, which bred guinea pigs for Huntingdon Life Sciences, and which had been the target of the animal rights campaign Save the Newchurch Guinea Pigs. The body was removed from a churchyard in Yoxall, Staffordshire and found buried in woodland on 2 May, 2006.
On 11 May, 2006, four people were convicted for their involvement in the incident, which was described in The Guardian newspaper as "a six-year hate campaign" that included letter bombs, vandalism, and grave robbing. The judge described the group's actions as "subjecting wholly innocent citizens to a campaign of terror." The campaign included hate mail signed Animal Rights Militia and Animal Liberation Front. Those convicted were Jon Ablewhite, John Smith and Kerry Whitburn each of whom who were given twelve year sentences and Josephine Mayo who was sentenced to four years. 
On 30 August 2007 ARM claimed to have deliberately contaminated 250 tubes of Novartis's widely-used antiseptic Savlon in shops including Superdrug, Tesco and Boots The Chemist who all withdrew sales of the cream. 
Singer, Peter. The Animal Liberation Movement: Its philosophy, its achievements and its future. Retrieved on 2007-11-09.
Best, Steven; Nocella, A John (2004-09-09). Terrorists or Freedom Fighters?: Reflections on the Liberation of Animals. Lantern Books. ISBN 159056054X.
a b c "Death-threat vivisector calls for industry support", BBC News, December 8, 1998.
"Staying on Target and Going the Distance: An Interview with U.K. A.L.F. Press Officer Robin Webb". No Compromise (22). Retrieved on 2006-05-23.