Parliament should get the final say on sending British troops to war, says shadow foreign secretary William Hague.
William Hague says moral authority is needed to fight terrorism
The Tory democracy task force has already been asked to look at the idea and Mr Hague told the party conference he backed the proposal.
Mr Hague said the UK had to learn from past mistakes to fight terrorism.
"It cannot now be doubted that the terrible failure to plan for the aftermath of the war in Iraq ... was one of them," he argued.
Mr Hague said "moral authority" was needed in the battle against terrorism. That will be seen as a renewal of his criticism of the Guantanamo Bay detention centre and alleged abuses of Iraqi prisoners.
He said the use of force must be a "last resort". And he echoed David Cameron's calls for the UK to avoid unquestioning support for American foreign policy.
"We need to know how we can manage an alliance with the United States that is not seen as one-sided; that is solid but never slavish," said Mr Hague.
"And how, as we face opponents who cannot be defeated by military force alone, we can use the greatest values of our free societies - our openness to fresh ideas and our respect for the rights of others - to inspire our friends and isolate our enemies, which means never besmirching those values by the abuse of prisoners or the abandonment of our own rule of law."