A classic war is unlikely to accidentally result in a such a set-back.
Yet if a group within a civilization has the very goal of restoring the entire population to an idealized version of an ancient way of life, they might succeed.
One rather recent historic example of such an attempt would be the Red Khmer with their leader Pol Pot, who strongly idealized an agriculture society and wanted to reset the country to the ways of the old people, resulting in a large scale genocide especially against their own Intelligentsia ("To keep you is no benefit, to destroy you is no loss.") and forced relocation of the people from the cities to rural areas and abolishment of schools.
They wanted to abolish everything modernized 20th century society requires and only were forcefully stopped by an intervention froma Vietnamese invasionintervention.
They wanted to build an agrarian socialist utopia.
In your setting, it could be a group of eco-terrorist/friends-of-nature forcing/helping to restore the planet to the ill-visioned/right way of life, who succeeded.