'Saar' was the French limited offensive into western Germany undertaken in an effort to support Poland soon after Germany had invaded the latter in 'Weiss' (i) (7/16 September 1940).

On 16/17 May 1939 there took place a series of talks between Général d’Armée Maurice Gustave Gamelin, the commander-in-chief of the French land and air forces, and Generał diwizji Tadeusz Adam Kasprzycki, the Polish war minister, about the increasing threat posed by Germany to both France and Poland. From this there emerged a protocol which mandated that as soon as part of the French forces became available (about the third day after France’s general mobilisation), France was to launch a series of progressive offensives with limited objectives, and that as soon as the main German attacks came to bear on Poland, France was to launch an offensive with the bulk of her forces (not earlier than the fifteenth day after France’s mobilisation).

Général d’Armée Alphonse Joseph Georges, commanding the French Groupe d’Armées de Nord-Est (north-eastern army group), intervened in the discussion to give 'some information on the Siegfried Line and on the artillery that will be needed to attack it', and his estimate was that 'to attack this line and achieve a breakthrough cannot be contemplated until the seventeenth day' after the French mobilisation.

Gamelin then declared that three-quarters of the French army could, even before any operations were launched by the Théâtre des Opérations du Nord-Est, undertake an offensive between the Rhine and Moselle rivers on the fifteenth day after mobilisation. But as far as the actual course of events in the Polish campaign was concerned, this meant the day before the USSR attacked Poland from the east.

The relevant documents do not show that the Polish war minister raised any objection to Gamelin’s programme. From this one can only conclude that Marszałek Polski Edward Śmigły-Rydz, the Polish commander-in-chief, and the majority of his French colleagues had absolutely no conception of either the paralysing effects of dive-bombing attacks or the imaginative use of armoured forces revealed by the Germans in 'Weiss' (i).

In addition to this paradoxical situation there was the incredible optimism of Gamelin himself, who opined that the Poles would hold out for at least six months and the French would therefore have time to come to their aid with forces shipped from France through the Mediterranean, Dardanelles, Bosporus and Black Sea to reach Romania.

'Saar' was laid down in a French army instruction of 24 July 1939, and was the first of the limited offensives mentioned by Gamelin. According to the Franco-Polish military convention, the French army was to start preparations for the major offensive three days after the start of mobilisation, and the French forces were to gain control over the area between the French border and the German lines, and also to probe the defences of General Erwin von Witzleben’s 1^st Army. On the fifteenth day after the start of mobilisation (16 September), the French army was to start a full-scale assault on Germany.

France’s pre-emptive mobilisation began on 26 August, and full mobilisation was ordered on 1 September. The French mobilisation process suffered from being an inherently obsolete system, and this had a decidedly adverse effect on the French ability to complete any swift deployment their forces into the field. The French command still believed in the tactics of World War I, which relied heavily on stationary artillery, even though this took considerable time to transport and deploy, and many pieces of artillery had also to be retrieved from storage before any advance could be made.

Directed by Général d’Armée André Gaston Prételat, commanding the 2ème Groupe d’Armées, the 'Saar' offensive in the Rhine river valley area was launched on 7 September, four days after the French declaration of war on Germany. At this time the German forces were fully occupied with their 'Weiss' (i) offensive in Poland, and the French troops had a potentially decisive numerical advantage along the border with Germany.

Despite this, all was stacked against the French including, as a first item, the fact that the frontier between the Rhine and Moselle rivers selected for the French attack had been defined by the victors of the Battle of Waterloo in 1815 with the express purpose of making French aggression difficult. Thus in 1939 the Germans held all the high ground. German-held salients extended into French territory, and would have to be reduced before the 'Siegfried-Linie' proper could be assaulted. In addition to this, the 'Siegfried-Linie' defences were sufficiently far to the rear of the German frontier defences that the French had to bring up their artillery to within easy range of German counter-battery fire.

After patrolling operations, Prételat launched his attack on 7 September, but the offensive advanced little past its start lines and took just 20 German villages, which had already been evacuated. A total of 31 divisions, including 14 first-line formations, had been put at Prételat’s disposal, but only nine of these were used. Général d’Armée Edouard Jean Réquin’s 4ème Armée, with its right flank in the Bitche region and its left in the Sarreguemines area on the Saar river, managed to capture 7.5 miles (12 km) of German territory as far to the north as a point just short of Breitfurt, while its neighbour, Général d’Armée Charles Marie Condé's 3ème Armée, pinched out the heavily wooded Warndtwald salient, comprising 3 sq miles (7.8 km²) of heavily mined German territory farther to the west in the area to the south-west of Saarbrücken.

von Witzleben’s 1^st Army had 17 divisions with which to meet this French attack, and 10 of these had been recruited only recently. The German troops made skilful use of their advantage in terrain, however, relying strongly on cleverly sited anti-tank and anti-personnel minefields. The French were unfamiliar with this threat and possessed no mine detectors. Houses booby-trapped with explosives added to the German minefield defences. The 'Siegfried-Linie' itself, which the French had planned to attack after 17 September, was described by General Siegfried Westphal as a 'gigantic bluff', but it was not. Its defences were sound, and the French artillery could do little damage to them. Generalmajor Ulrich Liss, head of the Fremde Heere West (foreign armies west) section of German army intelligence, stated that the French 155-mm (6.1-in) shells caused negligible damage. The shells of the heavier 220-mm (8.66-in) and 280-mm (11-in) guns were not fitted with the delay-action fuses which would have enabled the projectiles to penetrate the casemates before exploding. Liss conceded that the French guns maintained a high and accurate rate of fire, but stated that a large number of the French shells failed to explode as they came from stocks dating back to World War I.

This half-hearted 'Saar' offensive did not result in any diversion of German troops. The all-out assault was to have been carried out by some 40 divisions, including one armoured division and three mechanised divisions, 78 regiments of artillery with 4,700 pieces of artillery, and 40 tank battalions with 2,400 tanks against a German strength of 22 infantry divisions with only limited artillery and no armour.

On 12 September the Anglo-French Supreme War Council met for the first time at Abbeville in France, and decided that all offensive actions were to be halted with immediate effect. By that time the French divisions had advanced about 5 miles (8 km) into Germany along a front of only 15 miles (24 km) of the Franco/German border in the Saarland area. Gamelin ordered his troops to halt not less than 0.6 miles (1 km) from the German positions along the 'Siegfried-Linie'. Poland was not notified of this decision, and Gamelin informed Śmigły-Rydz that half of his divisions were in contact with the enemy, and that French advances had forced the Germans to withdraw at least six divisions from Poland.

The French losses were 22 men killed, 105 men wounded and 58 men missing, while those of the Germans were 196 men killed, 356 men wounded and 114 men missing, as well as 11 aircraft destroyed.

On the following day the head of the French military mission to Poland, Général de Division Louis Augustin Joseph Faury, informed the Polish chief-of-staff, Generał brygady Wacław Teofil Stachiewicz, that the planned major offensive on the western front had to be postponed from September 17 to September 20. At the same time, the French divisions were ordered to fall back to their barracks along the Maginot Line, so starting the period known as the 'Phoney War'.