The defence secretary has called for sweeping changes to international rules of war to counter global terrorism.
Mr Reid said the 20th century rules needed reassessing
John Reid warned the world was facing a threat from groups unconstrained by any sense of morality or convention.
"We risk trying to fight a 21st century conflict with 20th century rules," he said in a speech to the Royal United Services Institute.
He warned legal grounds for mounting pre-emptive strikes or intervening to stop genocide were no longer adequate.
Mr Reid also called for a review of the Geneva Conventions, signed in 1949, governing the treatment of prisoners of war.
He said the international legal framework governing rules of war had been drawn up at a time when the main international threat came from conflict between states and "did not contemplate the type of enemy which is now extant".
"The laws of the 20th century placed constraints on us all which enhanced peace and protected liberty. We must ask ourselves whether, as the new century begins, they will do the same," he said.
Mr Reid told the institute the world was facing a threat by unconstrained terrorist groups which were known to be trying to obtain weapons of mass destruction.
"We now have to cope with a deliberate regression towards barbaric terrorism by our opponents," he said.
"The legal constraints upon us have to be set against an enemy that adheres no constraints whatsoever, but an enemy so swift to insist that we do in every particular, and that makes life very difficult for the forces of democracy."