The Saar Offensive was the French invasion of Saarland, Germany, in the first stages of World War II, from September 7 to October 16, 1939, in response to the German invasion of Poland on 1 September 1939. The original plans called for 40 divisions, one armored division, three mechanised divisions, 78 artillery regiments and 40 tank battalions to assist Poland, which was then under invasion, by attacking Germany's neglected western front. Despite 30 divisions advancing to the border (and in some cases across it), the attack did not have the expected result. When the swift victory in Poland allowed Germany to reinforce its lines with homecoming troops, the offensive was halted. French forces then withdrew amid a German counter-offensive on 17 October.

In 1921, France and Poland made a defensive alliance against Germany through the military convention. France, along with the United Kingdom, declared war on Germany on September 3rd, 1939, two days after the German invasion of Poland.

According to the convention, the French Army was to start preparations for the major offensive three days after mobilisation started. The French forces were to effectively gain control over the area between the French border and the Siegfried Line and were to probe the German defences. The sector was defended by the German 1st Army. On the 15th day of the mobilisation (that is on 16 September), the French Army was to start a full-scale assault on Germany. The pre-emptive mobilisation was started in France on 26 August and on 1 September, full mobilisation was declared.

French mobilisation suffered from an inherently out-of-date system, which greatly affected their ability to swiftly deploy their forces on the field. The French command still believed in the tactics of World War I, which relied heavily on stationary artillery, even though this took time to transport and deploy. Many pieces also had to be retrieved from storage before any advance could be made.

Almost everyone expected a major French attack on the Western Front soon after the start of the war, but Britain and France were cautious as both feared large German air attacks on their cities; they did not know that 90 percent of German frontline aircraft were in Poland nor did they realise that the few German units that were holding the line had effectively been "pared to the bone" and stripped of any real fighting capability leaving the French unknowingly with a 3:1 advantage over the Germans. Consequently what followed was what historian Roger Moorhouse called a "sham offensive on the Saar" that began on 7 September, four days after France declared war on Germany. The Wehrmacht was engaged in the attack on Poland and the French enjoyed a decisive numerical advantage along the border with Germany but the French did not take any action that was able to assist the Poles. Eleven French divisions, part of the Second Army Group, advanced along a 32-kilometre (20 mi) near Saarbrücken, against weak German opposition. The French army advanced to as far as 8 km (5 mi) in some areas, and captured about 12 towns and villages with no resistance: Gersheim, Medelsheim, Ihn, Niedergailbach, Bliesmengen, Ludweiler, Brenschelbach, Lauterbach, Niedaltdorf, Kleinblittersdorf, Auersmacher, and Sitterswald (occasionally called "Hitlersdorf" in some French reports). Four Renault R35 tanks were destroyed by mines north of Bliesbrück.

By 9 September, the French occupied most of the Warndt Forest. On 10 September, while a minor German counterattack retook the village of Apach, French forces reversed the loss only hours later. The French 32nd Infantry Regiment made further gains on 12 September, seizing the German town of Brenschelbach with the loss of one captain, one sergeant, and seven privates. Near the meeting point of the French, German, and Luxembourgeois borders, the Schengen bridge was destroyed.

The offensive was halted after French forces had taken the 7-square-kilometre (2.7 sq mi) Warndt Forest, which had been heavily mined by the Germans. The French stopped short of the Siegfried line, although they came within a few kilometres south of it, immediately east of Saarbrücken.

The French held German territory along all of the Rhine-Moselle front, but after the collapse of Poland, General Maurice Gamelin on 21 September ordered French units to return to their starting positions on the Maginot Line. Some French generals, such as Henri Giraud, saw the withdrawal as a wasted opportunity and made known their disagreement with it.