More than 30 years after beginning an armed struggle, the Irish Republican Army began destroying its vast military arsenal on Tuesday, in effect declaring that its war for a united Ireland is over. It was a step the IRA repeatedly had proclaimed it would never take.

British Prime Minister Tony Blair hailed the breakthrough, which put a long-troubled peace process back on track, but he warned there would still be violence from "wreckers and cynics" who refuse to heed the desire of Northern Ireland's people for peace. He said Protestant paramilitary groups must give up their weapons as well and indicated Britain would respond by reducing its military presence in Northern Ireland as the IRA has long demanded.

Protestant Ulster Unionist leader David Trimble, who resigned in July as Northern Ireland's first minister in protest over the arms issue, described the IRA action as "clear evidence of a commitment" to the full implementation of the 1998 Good Friday peace agreement. He convened his party's 110-member executive to meet Saturday and said he would recommend that it endorse a return to a power-sharing government with Roman Catholics.

Trimble spoke after meeting members of the independent Decommissioning Commission, the body set up in 1998 to oversee disarmament. He said commission members had witnessed the action by the IRA in rendering a "substantial" amount of weaponry "permanently unusable and unavailable."

"The key thing is they were able to say this material will never, ever be used again," Trimble said.

The commission, headed by Canadian Gen. John de Chastelain, confirmed that it had witnessed the IRA "put a quantity of arms completely beyond use." That included weapons, ammunition and explosives, the commission said.

The commission wouldn't disclose how many weapons, how or when the process was carried out, or when the IRA might complete the disposal of an arsenal. It said revealing those details would not help further the disarmament process, an allusion to the fact the IRA action will not be welcome by some IRA members.

Security experts speculated that the IRA had destroyed part of its hidden arsenal either by using corrosive materials or concrete.

Politicians and commentators said they believed the Sept. 11 terrorist attack on the United States had been largely responsible for the IRA decision to disarm. They said it had become apparent to the IRA that it could not maintain its financial and political support among segments of the American population, newly sensitive to terrorist actions, unless it moved on the arms issue.

The IRA, other Catholic paramilitaries and Protestant groups have together been responsible for more than 3,000 deaths in the last 30 years. The IRA conceded in the late 1990s that the time had come to end its war, and it has maintained an official cease-fire for nearly four years although it and other paramilitary groups have never completely given up violence.

Sinn Fein, the Irish Republican Army's political wing, helped negotiate the April 1998 Good Friday peace agreement that brought Catholics and Protestants into a power-sharing government and to an understanding that Northern Ireland's constitutional status could only be changed by peaceful means.

Despite this, the IRA stubbornly resisted for years Protestant demands to yield its weapons, arguing that this would represent surrender in a war it felt it had not lost. IRA graffiti on walls in Belfast proclaimed, "Not One Bullet, Not One Ounce" of Semtex explosive.

But the IRA has allowed two of its arms dumps in the Irish Republic to be inspected by an international team several times in the past two years to demonstrate arms were not being used.

British security sources estimate the IRA possesses 3 tons of Semtex, about 1,000 rifles, 500 to 600 handguns and 1 million rounds of ammunition. It is also believed to own one SAM-7 surface-to-air missile and 40 rocket grenade launchers. Most of this weaponry was supplied by Libya in the 1980s, and security sources said they believe the IRA has arms caches in four counties in the Irish Republic.

Under heavy pressure within his own party, Trimble resigned as first minister in July to protest the IRA's lack of movement on disarmament and then pulled his party out of a power-sharing executive last week.

That left British Northern Ireland Secretary John Reid with no recourse except to suspend the Northern Ireland government by midnight Thursday unless the IRA started to give up its weapons. Reid had been expected to return Northern Ireland to direct rule by Britain.

The IRA issued a brief statement Tuesday afternoon saying it had started to disarm to save hopes for peace.

"The political process is now on the point of collapse," it said. "Such a collapse would certainly and eventually put the overall peace process in jeopardy.

"In order to save the peace process we have implemented this scheme. Our motivation is clear. This unprecedented move is to save the peace process and to persuade others of our genuine intentions."