A plan by Islamic extremists to target the English Defence League fell apart because the gathering in Dewsbury, West Yorkshire, finished earlier than expected.
Police and security services had no intelligence about the planned attack on June 30 last year, although one of the would-be killers was under surveillance in relation to another terrorist plan.
A gang of six men pleaded guilty to an attack that - but for a piece of good luck - would have resulted in large numbers of casualties.
A plot by six men from Birmingham to attack an EDL rally in Dewsbury was uncovered by "pure chance", police have said.
Six men accused of preparing to carry out an attack on a rally of the right wing group the EDL have pleaded guilty at Woolwich crown court.