The German occupation of Monsummano Terme began in the days immediately following the Armistice of 8 September 1943 and marked the beginning of all-out war in the area. A number of ordinances affected the area, compromising the lives of the local population: a curfew from 10 p.m. to 5 a.m.; the darkening of houses at night; the compulsory submission of straggling soldiers; a ban on aiding Anglo-American soldiers; the forced surrender of all weapons; a ban on listening to hostile radio channels. The situation of displaced persons was especially tragic at that time, as they were crammed into uninhabited facilities or housed in disused buildings, in poor sanitary conditions and often deprived of basic means of subsistence and clothing.

One of the most significant events was the arrest and deportation of Jewish citizens. On 5 November 1943 and in the days immediately following, a number of patrols organised by officers from the Montecatini public security police station and carabinieri from the Monsummano station, in collaboration with local German and republican soldiers, led to the imprisonment of people 'belonging to the Jewish race'. After a period of detention, ten Jews were transferred first to Florence, then to the Fossoli transit camp, finally arriving at Auschwitz, where they were murdered in the gas chambers.

The German influence in the village was seen through the work of the Todt organisation based in the Grotta Giusti spa; they recruited voluntary or forced civilian personnel to build barracks, concrete forts and tunnels. It was here, from the beginning of June until 14 July 1944, that the headquarters of Albert Kesselring, commander of the Wehrmacht in Italy, was located. He resided in Villa Renatico Martini, in a secluded position protected by the hills.

On 23 August 1944, after other massacres of civilians, 174 people were murdered during the Padule di Fucecchio massacre. The massacre was carried out by sections of the German army's 26th Armoured Division and included children, women, and the elderly, many of them farmers or displaced persons. The aim of the operation was to clear the territory was to eliminate any human presence that could help the partisans and hinder their retreat towards the Gothic Line, which ended definitively a few days later. The town was liberated by the partisans on 4 September 1944.

Monsummano Terme was decorated with the Silver Medal of Civil Merit.