The SDLP has dramatically moved into opposition at Stormont and is now set to join forces with the Ulster Unionists.
But SDLP leader Colum Eastwood pledged it would be constructive, progressive and would not amount to "opposition for opposition's sake".
For the party whose former leader John Hume is regarded as the chief architect of the Good Friday Agreement, it was not an easy decision.
But it followed a meeting with First Minister Arlene Foster and Deputy First Minister Martin McGuinness to discuss SDLP demands for changes to the Programme for Government (PfG) which lasted only half an hour.
The move came eight days after UUP leader Mike Nesbitt upstaged the reappointment of Mrs Foster and Mr McGuinness by announcing his party would form an opposition, even before talks on the PfG had begun.
SDLP leader Colum Eastwood said it became apparent the DUP and Sinn Fein were not prepared to bring about changes including stopping the continuing tide of emigration and improved investment in education.
"It has become clear that our ambition for a full Programme for Government will not be matched by the document currently constructed by the DUP and Sinn Fein. We fear its inherent vagueness will fall far short of what is required," he said.
But he also added: "Entering into opposition is not an easy decision to make, particularly for a nationalist party in the North.
"Since partition, our community was long denied power in this very building and therefore we have long been in opposition. That memory runs deep.
"But those were different days and this is a different Ireland."
The Foyle MLA said the decision had the unanimous support of the party's executive and Assembly group and would last for the entire period of the new Assembly term, until 2021.
Addressing a Press conference in the Great Hall at Parliament Buildings, Mr Eastwood, whose party lost two seats in the Assembly election earlier this month, said: "We have made a bold decision that hasn't come lightly to us, but... it will not be opposition for opposition's sake, it will be positive and constructive."
The SDLP chief said as builders of the institutions, the party had been reluctant to leave the responsibilities of government to parties that "hindered" their operation.
"It was a reluctance to give the keys of the house to those who had very little part to play in laying that house's foundation, never mind in actually building it, brick by brick," he said.
"For the good of our politics, the SDLP is now breaking free from that past reluctance." Mr Eastwood said it had never been intended that the Good Friday Agreement should be frozen in time. "Change is as constant a feature in politics as it is in life," he said.
UUP leader Mr Nesbitt said he was confident the move "will lead to new beginnings and possibilities for devolved government."
The former chair of the Assembly committee which monitored Mrs Foster and Mr McGuinness said he had enjoyed working with Mr Eastwood on the committee.
"I very much look forward to working in partnership as we bring on this new era," he said.
"Be in no doubt, together we can offer the opportunity of a real fresh start."