'Treffenfeld' was a German and Vichy French operation against French resistance forces of the Maquis de l’Ain and Maquis du Haut-Jura in the south-eastern part of German-occupied France (11/21 July 1944).

By a time early in the summer of 1944, the Germans feared that the Allies might undertake an amphibious assault on the coast of Provence in the south-eastern part of occupied France, currently the area controlled by Generaloberst Johannes Blaskowitz’s Heeresgruppe 'G' with General Joachim Lemelsen’s (from 4 June General Kurt von der Chevallerie’s) 1^st Army (one Panzer, one Panzergrenadier and four infantry divisions) in south-western France and General Georg von Sodenstern’s (from 29 June General Friedrich Wiese’s) 19^th Army (two Panzer and nine infantry divisions) in south-eastern France. As well as the basic threat posed by an Allied amphibious assault on the Provence coast, which was held by four infantry divisions, of which one was refitting, the Germans had to take into consideration the additional danger of the major French resistance forces operating independently on in co-operation with an Allied landing.

The Germans therefore undertook two major operations in an effort to break the strength and cohesion of the resistance forces behind the possible landing areas: these undertakings were 'Bettina' against the Maquis du Vercors, and 'Treffenfeld' against the Maquis de l’Ain et du Haut-Jura, the latter in the central and southern parts of this area to the east of the Rhône river abutting the Franco/Swiss frontier to the west of Lake Geneva.

Under the overall supervision of General Heinrich Niehoff, commander of the southern France army area, and immediate control of Generalleutnant Wilhelm Hederich, the military commander of north-eastern France with his headquarters in Dijon, the forces assembled for the undertaking were between 9,000 and 10,000 men of the 1^st Reserve-Gebirgsjägerregiment of Generalleutnant Karl Pflaum’s 157^th Reserve-Division, various Osttruppen (including Cossack) units of Generalmajor Wilhelm von Henning’s Freiwilligen Stamm-Division which had been raised in southern France during January 1944 to train cadres for the volunteer units which the Germans were raising from Soviet prisoners of war, men of the officers' school in Dijon, a few Luftwaffe ground units, Sicherheitsapolizei and Sicherheitsdienst elements, and Vichy French Milice Française para-military police/militia units.

Thus as many as 10,000 men were used in the operation, a figure twice as great as those used during the earlier 'Korporal' and 'Frühling', in the southern and central parts of the Jura area on 5/13 February 1944 and 7/18 April 1944 respectively, as despite these undertaking operations and since the time of 'Overlord' the strength of the French resistance forces had been growing rapidly.

The undertaking’s main objectives were to retake the towns of Nantua and Oyonnax, which were currently controlled by the resistance, and to free German soldiers currently being held by the resistance. From the start, the Germans decided that they would use any and all means to encompass the defeat of the Maquis de l’Ain et du Haut-Jura, these methods including the killing or raping of people of the civil population and the looting and burning of homes. The German progress into the resistance-held area was to involve the advance of columns of the Freiwilligen Stamm-Division from the north and south as the hammers to drive the resistance fighters eastward onto the anvil of the mountain infantry battalions of the 157^th Reserve-Division.

On 8 July elements of the 157^th Reserve-Division began preliminary operations in the area of Artemare to regain control of a tactically important position located at the entrance of a tunnel some 1.25 miles (2 km) from Virieu le Grand, which had been taken on only the previous day by the resistance. The next preliminary assault began on 9 July to the north and west of Artemare, extended on the following day to the area of Champagne en Valromey with the object of taking the Col de Richemond. For the imminent main phase of the operation, the German command post was located first at Culoz but then moved to Artemare.

On 9 July elements of the Freiwilligen Stamm-Division started to arrive around Bourg en Bresse, and at this stage the Germans began a secondary task, namely the seizure of some 1,280 men aged between 17 and 45 as forced labour for use in Germany, but in response to a request from the prefect of the Ain region and German fears that a general uprising might result, this forced labour raid was suspended on 13 July to allow greater impetus to be given to the purely military operations.

'Treffenfeld' proper lasted from 11 to 21 July, and began after an additional regiment had reached Artemare. The first priority was the seizure of the Hauteville and Nantua areas, and on 12 July the German forces gained control of the Col de Richemond, Hotonnes, Ruffieu and the Col de Rochette. At the same time one battalion held the line between Bellegarde sur Valserine and St Germain de Joux to block the resistance forces' possible line of retreat to the north. The local resistance leader, Henri Romans-Petit, was all too aware of his forces' numerical and matériel inferiority to the German forces, and therefore decided to avoid direct combat. Instead, Romans-Petit organised sabotage operations against German convoys in order to slow the German advance and buy time for the resistance forces to withdraw. Another resistance objective was to retain areas earmarked for the 'Cadillac' air-drop of weapons, which was scheduled for 14 July, but Romans-Petit then came to appreciate the impossibility of protecting the designated area and cancelled the element of 'Cadillac' scheduled for the Département de l’Ain. On 12 July resistance forces under Henri Girousse and the Freiwilligen Stamm-Division clashed at Pont d’Ain, to the advantage of the German force. Girousse ordered his men to fall back, and to slow the German advance ordered the Verduraz and Louison resistance groups to establish an ambush on the plateau of Hauteville-Lompnès along Highway 84. The resistance’s southern group fell back into the Forêt d’Arandas.

The resistance’s northern group now had to establish and maintain the defence of the area round Oyonnax. German forces reached Dijon , Dole and Besançon, and tried to encircle the resistance group by advancing through Oregelet, Thoirette and Izernore on the one hand, and St Claude, Dortan and Oyonnax on the other. Another resistance leader, Noël Perrotot, deployed units in the area as part of his plan to hold the area until the order to fall farther back was given at 22.00 on 12 July. The German advance had slowed but not come to a halt, and on 12/13 July the 5^th Don-Kosaken-Reiterregiment perpetrated a massacre of 36 persons at Dortan, and then took Oyonnax on 14 July.

In the east of the area, a section of the Lorraine company under the command of Léon Boghossian and a local force of the Armée Secrète (an amalgamation of the smaller Combat, Libération-Sud and Franc-Tireur units) led by André Lamblot sought to contain the 157^th Reserve-Division at St Germain de Joux, but the Germans used member of the local population as 'human shields' and were therefore able to continue their advance. Henri Romans-Petit instructed André Lamblot’s force, which was now threatened with encirclement, to retreat.

The resistance’s western group was responsible for intercepting and if possible halting German convoys from Bourg en Bresse, and opted to use the Ain river as its line of defence. On 11 July Germans troops threatened Neuville sur Ain and the valley of Suran river, and the resistance force placed ambushes along the German line of advance, but were then ordered to fall back, in the process demolishing part of the Cize viaduct and the bridge at Serrières sur Ain.

Messages from Denis Johnson, who had replaced Richard Heslop as the Special Operations Executive’s liaison officer with the Maquis de l’Ain, made it possible for the staff of the Forces Françaises de l’Intérieur (French Forces of the Interior) in London to follow the course of the fighting as it took place, and soon learned that the German attack was forcing the resistance units to fall back into the mountains. Johnson asked for a British air attack on the airfield at Ambérieu, and on 16 July a raid by six bombers destroyed 42 Messerschmitt aircraft.

In the middle of July, weapons including machine guns and mortars, were paradropped into the area of Echallon and Chartreuse de Portes. The first delivery was made on the night of 14/15 July, but the warning message was transmitted in clear and the Germans knew what was about to happen. The resistance was not able to call off the delivery and, despite the blocking of all the approach roads by the Germans, resistance groups designated to recover the containers managed to secure 64 of them by 08.00 on 15 July. It was impossible for the resistance groups to remove all trace of their activities in the night, however, and a force of 500 Germans located one resistance group. In the fighting that followed, the resistance lost 15 men but escaped with 55 containers, and in retaliation the Germans burned 12 farms.

At the end of their operation in the Ain region, the two German divisions redeployed to the Vercors massif, where another major resistance effort was brewing, and left no units in the areas round Oyonnax and Nantua. The populations of these two small towns, which were those most directly affected by 'Treffenfeld', felt that the resistance had abandoned them during the operation and, feeling that the resistance would not reappear locally, resorted to a level of looting that led to new German operations. At Bellegarde sur Valserine local sentiment was more moderate, but did not favour any return by the resistance. At Ambérieu en Bugey, however, the population favoured a return of the resistance as sabotage of local railway installations had prevented Allied bombing raids.

During 'Treffenfeld' the Germans claimed to have killed 450 people, including a number of civilians. The FFI admitted the loss of 85 persons killed and another 80 wounded. 'Treffenfeld' had seen the use of three times more German soldiers than in 'Korporal', and the number of civilians executed was four times greater and the number of homes destroyed six times greater.