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Sen. Bernie Sanders is well-known for his longtime opposition to America's prison-industrial complex and prisoner abuse. But the Vermont socialist and political firebrand also has a history of advocating for prisoners' rights internationally, like he did in 1981. That was the year when he — then as mayor of Burlington, Vermont — sent British Prime Minister Margaret Thatcher a scathing letter about the abuse of Irish prisoners in the U.K.

The letter scolds Thatcher for British treatment of hunger-striking prisoners who were members of the Provisional Irish Republican Army. At the time, the U.K. and Ireland were deeply enmeshed in "The Troubles" — a long-simmering civil conflict between Irish nationalists on one side and the Brits and loyalists to the Crown on the other — and treatment of imprisoned IRA members became a symbol of the broader conflict.

"We are deeply disturbed by your government's unwillingness to stop the abuse, humiliation and degrading treatment of the Irish prisoners now on hunger strikes in Northern Ireland," Sanders wrote. "We ask you to end your intransigent policy towards the prisoners before the reputation of the English people for fair play and simple decency is further damaged in the eyes of the people of Vermont and the United States."

In 1981, IRA prisoners began a prolonged hunger strike that ultimately took the lives of 10 prisoners, including that of Bobby Sands, who had been elected to a seat in Parliament while in prison. The strikes eventually ended after the death toll began to climb and riots broke out across Ireland. Shortly after, some concessions were made to the prisoners.