Dublin - Martin McGuinness, the Irish Republican Army commander who led his underground, paramilitary movement toward reconciliation with Britain, and was Northern Ireland's deputy first minister for a decade in a power-sharing government, has died, his Sinn Fein party announced Tuesday on Twitter. He was 66.The party said he died after a short illness. He suffered from amyloidosis, a rare disease with a strain specific to Ireland's northwest. The chemotherapy required to combat the formation of organ-choking protein deposits quickly sapped him of strength and forced him to start missing government appointments.McGuinness's transformation as peacemaker was all the more remarkable because, as a senior IRA commander during the years of gravest Catholic-Protestant violence, he insisted that Northern Ireland must be forced out of the United Kingdom against the wishes of Protestants.Even after the Sinn Fein party - the IRA's legal, public face - started to run for elections in the 1980s, McGuinness insisted as Sinn Fein deputy leader that "armed struggle" remained essential."We don't believe that winning elections and any amount of votes will bring freedom in Ireland," he told a BBC documentary team in 1986. "At the end of the day, it will be the cutting edge of the IRA that will bring freedom."Yet within a few years of making that stubborn vow, McGuinness was exploring the opposite option in covert contacts with British intelligence that led eventually to a truce, inter-party talks and the installation of the IRA icon in the heart of Northern Ireland's government.'A mass killer'Irish Times columnist Fintan O'Toole argued in January 2017 that McGuinness had been "a mass killer - during his period of membership and leadership the IRA killed 1 781 people, including 644 civilians - whose personal amiability has been essential to the peace process. If he were not a ruthless and unrepentant exponent of violence, he would never have become such a key figure in bringing violence to an end."Unlike his close Belfast associate, Sinn Fein leader Gerry Adams, McGuinness never hid the fact that he had been a commander of the IRA - classed as a terrorist organisation by the British, Irish and US governments. Nor could he.Born May 23, 1950, he joined the breakaway Provisional IRA faction in his native Londonderry - simply Derry to Irish nationalists - after dropping out of high school and working as an apprentice butcher in the late 1960s. At the time, the Catholic civil rights movement faced increasing conflict with the province's Protestant government and police.He rose to become Derry's deputy IRA commander by age 21 as "Provo" bombs systematically wrecked the city centre. Soldiers found it impossible to pass IRA road barricades erected in McGuinness's nearby Bogside power base.McGuinness appeared unmasked at early Provisional IRA press conferences. The BBC filmed him walking through the Bogside discussing how the IRA command structure worked and stressing his concern to minimise civilian casualties, an early sign of public relations savvy. Sinn Féin Leader and former Deputy First Minister Martin McGuiness dies after short illness https://t.co/OvB8BMQhxZ pic.twitter.com/1xlbkFqiMt— Sinn Féin (@sinnfeinireland) March 21, 2017 ArrestIn 1972, Northern Ireland's bloodiest year, McGuinness joined Adams in a six-man IRA delegation flown by the British government to London for secret face-to-face negotiations during a brief truce. Those talks got nowhere and McGuinness went back on the run until his arrest on New Year's Eve in the Republic of Ireland near a car loaded with 110 kilograms of explosives and 4 750 rounds of ammunition.During one of his two Dublin trials for IRA membership, McGuinness declared from the dock he was "a member of the Derry Brigade of the IRA and I'm very, very proud of it".Historians and security analysts agree that McGuinness was promoted to the IRA's ruling army council following his November 1974 parole from prison and would have overseen many of the group's most spectacular and divisive attacks. These included bomb attacks on London tourist spots and the use of "human bombs" - civilian employees like cooks and cleaners at British security installations - who were forced to drive car bombs to their places of work and were detonated by remote control before they could raise the alarm.His central role in the IRA command was underscored when Britain in 1990 opened secret dialogue with the underground group in hopes of securing a ceasefire. An MI6 agent codenamed "The Mountain Climber" met McGuinness several times as part of wider diplomatic efforts that delivered a 1994 IRA truce and, ultimately, multi-party negotiations on Northern Ireland's future and the US-brokered Good Friday peace accord of 1998.Northern Ireland's first power-sharing government, formed in 1999, was led by moderates and afforded only minor roles for Sinn Fein and the most uncompromising Protestant party, Paisley's Democratic Unionists. When Sinn Fein nominated McGuinness to be education minister, many Protestant lawmakers recoiled and insisted they would never accept what one called "an IRA godfather" overseeing their children's education.That first coalition collapsed under the twin weight of Paisley-led obstruction and the IRA's refusal to disarm as the Good Friday pact intended. McGuinness served as the lead liaison with disarmament officials.After election results vaulted the Democratic Unionists and Sinn Fein to the top of their communities for the first time, pressure mounted on the IRA to surrender its stockpiled arsenal. This happened in 2005, paving the way for Paisley to bury the hatchet with the group he called "the Sinners".Unlikely friendship No observer could have foreseen what happened next: a genuine friendship between First Minister Paisley and Deputy First Minister McGuinness. Belfast wits dubbed them "The Chuckle Brothers" because of their public warmth, an image that quickly eroded Protestant support for Paisley and forced him out as Democratic Unionist chief within the year.McGuinness maintained more businesslike relations with Paisley's frosty successor, Peter Robinson. Together they met Queen Elizabeth II for a historic 2012 handshake in Belfast and were guests of honour at Windsor Castle two years later. All the while, McGuinness expressed newfound support for the police as they faced attacks from IRA splinter groups - a U-turn that exposed McGuinness and his relatives to death threats in their Derry home.His relations with the newest Democratic Unionist Party leader, Arlene Foster, turned sour with surprising speed. When Foster rebuffed Sinn Fein's demands to step aside, McGuinness resigned in January, toppling power-sharing in the process."Over the last 10 years I have worked with DUP leaders and reached out to unionists on the basis of equality, respect and reconciliation," a frail, weak-voiced McGuinness said as he resigned as deputy first minister. "Today is the right time to call a halt to the DUP's arrogance."McGuinness is survived by his wife, Bernadette, two daughters and two sons.