During the German invasion of May 1940, the SOMUA S35 proved itself to be a tactically effective type, but this was negated by the French command's strategic mistakes in deploying their Cavalry armoured divisions.

After the defeat of France in June 1940, limiting production to a total of about 440, captured SOMUA S35s were used by the Axis powers, some of them on the Eastern Front.

A derived type, the SOMUA S40, with an improved suspension, lowered hull cast and welded turret armour, had been planned to replace the original version on the production lines in July 1940.

Agreements to produce this improved type for the benefit of Vichy France, Germany, and Japan, ultimately did not lead to any manufacture.

The design of the SOMUA S35 comes from the changed specifications of 26 June 1934 for an Automitrailleuse de Combat (AMC) issued for cavalry use.

[1] By 17 May the Army had already contacted a subsidiary of Schneider et Cie — the Société d'Outillage Mécanique et d'Usinage d'Artillerie (SOMUA) based at Saint-Ouen — to build a prototype.

On 25 March 1936, the AC4 design was accepted as the standard medium tank of the French Cavalry with the official name Automitrailleuse de Combat modèle 1935 S (AMC 1935 S), when a first order for fifty was made.

[3] The turret was a variant of the APX 1 as used on the Char B1: the APX 1 CE (chemin élargi) with a larger 1,130 mm (44 in) as against 1,022 mm (40.2 in)) turret ring, allowing the radio operator to assist the commander in loading the gun from an ammunition stock of 118 shells (90 AP, 28 HE) and 2,250 machine gun rounds.

As with the B1, the commander was expected to direct the tank while also loading, aiming, and firing the 47 mm SA 35 main gun — although at least the radio duty could be left to another crew member.

The nominally 200 hp engine, designed by Javier-Sabin, drew fuel from the smaller tank, which was itself automatically replenished from the larger one.

[6] The French Army preferred to fight a defensive battle, and was convinced it would be victorious in such an engagement, but understood that the strategic situation might impose the necessity of waging offensive warfare; during the Second World War for 1941 large offensives against Germany were planned, based on an expected French and British material superiority.

[7] Each of the two phases of an offensive – the breakthrough by the Infantry and the exploitation of it by the Cavalry – called for its own specialised vehicle, with the AMC designed to fight enemy armour.

[8] So it could carry out deep strategic penetrations and destroy enemy armour reserves trying to prevent them, possessing a good anti-tank capacity.

A commander was supposed to acquire such a degree of dexterity that his workload did not negate the lack of need to coordinate the actions of three men in a larger turret crew or the advantage of a quicker reaction because of a superior rotation speed.

[citation needed] The Army branches issued very vague specifications, leaving it to private enterprise to come up with precise proposals.

The French machine tool national stock was relatively outdated and tank designs reflected the limited existing production facilities.

To introduce a Christie suspension — the obvious solution[citation needed] — demanded a thorough industrial modernisation and the raising of quality standards.

[13] The strategic flaw was the high unit price of the tank, 982,000 French Francs,[14] and the limited number of large cast sections that could be produced.

As the French production capacity for cast armour sections was insufficient, at the time of the defeat negotiations were ongoing to employ American producers, and during the crisis caused by the German attacks in June it was even proposed to let US firms build two thousand vehicles.

[1] These vehicles were later issued to several ad hoc units, such as the 4th DCR (commanded by Charles de Gaulle) which received 39, part of 3e Cuirassiers, the 4th DLM (10), and some Corps-francs Motorisés (about 25).

Also the destroyed 1st, 2nd and 3rd DLM were reconstituted with a small number of tanks, the first two divisions received ten S 35s, the third twenty; S 35s further served with the 7e Cuirassiers (25) and a platoon of three was present in the 3e RAM of the 3e DLC.

[17] In May 1940 during the Battle of France the DLMs were tasked with the difficult manoeuvre of carrying out a quick advance into the Low Countries, followed by a holding action to allow the infantry divisions following behind to dig themselves in.

Both types of units were very similar in equipment, training and organisation, as the German armoured divisions too were primarily intended for strategic exploitation, while the breakthrough phase was preferably left to the infantry.

The resulting disorder and breakdown of most of its S 35s rendered this unit, the most powerful of all Allied divisions, impotent; it was defeated by the German 5th Panzerdivision on 17 May.

[21] After taking part in the Tunis victory parade, 12e RCA's S35s were replaced by M4 Shermans, but crews often affixed the SOMUA plate on their new tanks.

[23] French soldiers used SOMUA tanks in the attack on the German-held Oléron Island on the west coast of France in April 1945.

Panzerregiment, which battalion was renamed Panzer-Abteilung 211 and sent to Finland in June,[25] the only major German unit with S 35s that would fight on the Eastern Front; some were deployed by 22.

[14] The armament and general nominal armour base would remain the same, but first steps to improve these, which might have naturally have evolved into a "S 41", had already been taken in the spring of 1940, when plans were made for a 60 mm welded ARL turret.

[36] In early 1942, France was finally given permission to reequip its forces with SOMUA S40s, now that the type had become obsolete and the Germans had lost interest.

[37] However, a clandestine development continued, of a SARL 42, fitted with the ARL 3 turret and a 75 mm L/32 or L/44 gun using an optical range finder.