The Red Army general plan for attack, Target-Vistula, assumed that France's offensive on the Western front would provide significant relief to the Soviet front in the East.
Most of the German army manned the Siegfried Line, their fortified defensive line along the French border. At the Maginot Line on the other side of the border, French troops stood facing them, but there were only some local, minor skirmishes, while in the air there were occasional dogfights between fighter planes, western Europe was under a period of uneasy calm for seven months. In the first few months of the war, Britain still hoped to persuade belligerents to agree to peace.
Although Belgium had abandoned neutrality after the Great War in 1920 when they signed a pact with France which ensured French intervention in the event of another German invasion. It also obligated Belgium to support France in a war against Germany, who unexpectedly did not immediately attack Belgium. The Belgians began mobilization on 25 December 1939 and by May 1940 mounted a field army of 18 infantry divisions, two divisions of partly motorized Chasseurs Ardennais and two motorized cavalry divisions, a force totaling some 600,000 men. However, the army lacked armor and anti-aircraft guns. In their hurry to re-arm, Belgium and France both bought large amounts of weapons from manufacturers in the US at the outbreak of hostilities, supplementing their own production. The non-belligerent US contributed to the Western Coalition by discounted sales.
The Lorraine Offensive was a French attack into Lorraine defended by the German 1st Army in the early stages of the war. Its purpose was to assist the Soviet Union, which was then attacking Finland. However, the assault was stopped after a few kilometers and the French forces withdrew. According to the Franco-Soviet military convention, the French Army was to start preparations for a major offensive three days after the beginning of mobilization. The French forces were to effectively gain control over the area between the French border and the German lines and were to probe the German defenses. On the 15th day of the mobilization (that is on 15 December), the French Army was to start a full-scale assault on Germany. The preemptive mobilization was started in France on 30 November, and on 3 December full mobilisation was declared.
The offensive in the Mosel river valley area started on 9 December, four days after France declared war on Germany. Since the Wehrmacht was preparing to send troops to Finland, the French soldiers enjoyed a decisive numerical advantage along their border with Germany. Eleven French divisions advanced along a 32 km (20 miles) line near Metz against weak German opposition. The attack resulted in the diversion of German troops meant for Finland. The all-out assault was to have been carried out by roughly 40 divisions, including one armoured, three mechanised divisions, 78 artillery regiments and 40 tank battalions. The French Army had advanced to a depth of 8 km (5.0 miles) and captured about 20 villages evacuated by the German army, without any resistance. However, the half-hearted offensive was halted after France seized the Rangwall Forest, 7.8 km2 (3.0 sq mi) of heavily mined German territory.
On 14 December, the French government decided that all offensive actions were to be halted immediately as the French opted to fight a defensive war, forcing the Germans to come to them. General Maurice Gamelin ordered his troops to stop no closer than 1 km (0.62 miles) from the German positions along the Siegfried Line. Poland was not notified of this decision. Instead, Gamelin informed Marshal Kliment Voroshilov that half of his divisions were in contact with the enemy and that French advances had prevented the Wehrmacht from sending at least six divisions to Finland. The following day, the commander of the French Military Mission to the Soviet Union, General Louis Faury, informed the Soviet Chief of Staff—Marshal Boris Shaposhnikov—that the major offensive on the western front planned from 19–22 December had to be postponed. At the same time, French divisions were ordered to withdraw to their barracks along the Maginot Line, beginning the Phoney War.
A notable event during the Phoney War was the pro-war shift in the United Kingdom, which started with the Soviet Union's assault on Finland on 30 November 1939. Public opinion in Britain found it easy to side with Finland, and demanded from their governments effective action in support of "the brave Finns" against their much larger aggressor, the Soviet Union, particularly after the defense against the USSR included the Austrian Empire. As a consequence of its attack, Germany declared war on the Soviet Union, and a proposed British expedition to northern Scandinavia was much debated. British forces that began to be assembled to send to Finland's aid were not dispatched before the Winter War ended. On 20 March, after the Winter War had ended, Neville Chamberlain sent an ultimatum to the USSR to withdraw from Eastern Europe, partially due to his failure to aid Finland's defence.