After an initially quiet reaction by the French media, President Jacques Chirac set off a furore when he dismissed the Prime Minister's words as "ill-timed" on Friday evening, 24 hours after the original speech.
Since then, other right-wing politicians and veterans' groups have unleashed salvoes of insults against the Socialist Prime Minister, accusing him of pacifism, rewriting history, honouring cowardice and insubordination and (perhaps his only true offence) usurping President Chirac's role as the guardian of the national conscience.
On Thursday, Mr Jospin became the first French political leader for 81 years to visit the Chemin des Dames battlefield in the Aisne, scene of a disastrous offensive in April and May 1917, which produced widespread mutinies in the French army. According to differing historical accounts, between 49 and 75 soldiers were executed for their part in these mutinies, repressed by General Philippe Petain, the hero of Verdun and, later, leader of the collaborationist Vichy regime during the 1939-45 war.
Mr Jospin, whose father, Robert, was a pacifist activist between the wars, seemed to have taken a personal decision to revisit a painful and long- taboo subject, as France prepared for Wednesday's 80th anniversary of the end of the Great War. Speaking on the Chemin des Dames battlefield on Thursday, he said it was time to bring those men "shot as an example ... fully back into our collective, national memory".
He said they had been victims of orders that threw them against an "impregnable objective. Exhausted, condemned in advance, sliding into mud soaked in blood, plunged into bottomless despair, they refused to be sacrificed." It was time to remember them - and 3,000 other men condemned to lesser penalties - as victims of the war like any other soldiers.
Reaction in the French press next morning was muted. It appears that even Mr Chirac at first saw nothing much to complain about in Mr Jospin's words. It was only when the influential evening newspaper Le Monde splashed on the speech later that day, with the headline "The Republic honours the mutineers of 1917", that the Elysee decided that it must react.
That evening, the President said: "At the moment when the nation is commemorating the sacrifice of more than a million French soldiers who gave their lives between 1914 and 1918 to defend the country from the invader, the Elysee judges ill-timed any declaration which can be interpreted as a rehabilitation of mutineers."
French press commentary suggests that Mr Chirac was partly motivated by pique that Mr Jospin had broken the informal rules of their cross-party co-habitation by not informing him of his speech in advance.
By talking of the "collective national memory" the Prime Minister had also - perhaps deliberately - strayed into one of the areas of public life reserved for the President.
Other centre-right politicians are said to have brought pressure on Mr Chirac to use the "pardon" speech as an opportunity to slap down the popular Mr Jospin as part of a more aggressive approach to the right-left cohabitation in government.
The 1917 mutinies have always been a sensitive issue in France. The 1958 Stanley Kubrick movie Paths of Glory, about executions of French troops earlier in the war, was banned in France until 1976.
Mr Chirac's neo-Gaullist RPR party said that if all soldiers had followed the mutineers, the "war would have been lost and our people subjected to the yoke of empires".
Francois Hollande, first secretary of the Socialist Party, pointed out that the executed soldiers had not run away: they had simply refused to take part in further, futile offensives. "They were not bad Frenchmen but ordinary men at the end of their tether, plunged into a hell of blood and fire."