The Piaggio-Valmara customs were occupied by German forces belonging to the Zollgrenzschutz from September 1943.
As the weeks passed, they were replaced by the GNR Milizia Confinaria, a fascist unit established in the autumn of 1943 with the mission of controlling the border area. These areas saw significant involvement in military activities during the initial phases of the reoccupation of the partisan "Free Zone" in Ossola. During this period, on October 10, 1944, there were significant clashes right at the border. Not so much between Germans and partisans, but rather between Germans and the fascists themselves. In fact, due to poor communication among them, the Germans fired upon the fascists, thinking they were partisans.
The consequences of this clash led to the temporary exodus to Switzerland of almost the entire population of San Bartolomeo Montibus, and several dozen civilians sought temporary refuge in Brissago to escape the heavy combat. These battles resulted in the serious injury of the commander of the SS-Pol.Rgt.15 battalion, Major Hartell.
After the German reoccupation of the "Partisan Republic" of Ossola, the military presence at the border diminished due to logistical shortages and organizational issues. The very few militiamen still present by the end of April 1945 preferred to desert to Switzerland rather than be captured by the partisans.