The British Prime Minister has gambled on the future of the political process in the North by offering full and swift implementation of the Belfast Agreement in return for an end to IRA and all paramilitary activity.
In a lightning visit to Belfast to deliver a speech which was surprising for its bluntness and audacity, Mr Blair said politics had reached a fork in the road and it was time for crunch decisions on all sides.
Avoiding the sensitive term "disbandment" he said: "We cannot carry on with the IRA half in, half out of this process. Not just because it isn't right any more. It won't work any more."
The peace process would be unstoppable if the threat of violence was removed. "That threat, no matter how damped down, is no longer reinforcing the political, it is actually destroying it." The way ahead, he said, lay in "acts of completion" by the governments, nationalists, unionists and paramilitaries.
"Should real change occur, we can implement the rest of the agreement, including on normalisation, in its entirety and not in stages but together."
The Sinn Féin response was measured. Mr Gerry Adams urged Mr Blair to "go beyond the rhetoric". "He admits that his government has yet to complete its obligations," he said. "They need to give us their plan for implementation."
Earlier, Mr Martin McGuinness said implementation of justice and policing reforms would pose an enormous challenge to paramilitaries. "The key to it is to ensure that the agreement is implemented in full. The challenge that we then pose to all our armed groups within this society is an absolutely mighty one," he said.
In a statement, the Taoiseach, Mr Ahern, welcomed "the Prime Minister's robust statement that the British government will not countenance any path other than implementing the agreement and its willingness to give whatever guarantees are required in that respect".
He pledged to work closely with Mr Blair "in achieving the quantum leap forward that is now required." It was time to move on "acts of completion", he added.
In a deeply personal and analytical address to an invited audience in Belfast's Harbour Commissioner's office, Mr Blair explained why he believed the agreement was the only framework which could accommodate Northern Ireland's political and constitutional needs.
However, in remarks aimed at republicans, Mr Blair said the retention of the capacity for republican violence was now wrecking the process.
Mr Ahern echoed these remarks. The Taoiseach said the retention of "a parallel track" was "not just wrong in itself, but also obstructs the achievement of the republican movement's own programme".
Mr Blair also posed a question for Mr Ahern's government. "To this blunt question: how come the Irish Government won't allow Sinn Féin to be in government in the South until the IRA ceases its activity, but unionists must have them in government in the North?, there are many sophisticated answers. But no answer as simple, telling and direct as the question."
He confirmed he would not renegotiate the Belfast Agreement and insisted: "I honestly believe there is no other way".