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, and 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, 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.

The Invasion of Poland, also known as the September Campaign, Polish Campaign, War of Poland of 1939, and Polish Defensive War of 1939, was a joint attack on the Republic of Poland by Nazi Germany, the Slovak Republic, and the Soviet Union; which marked the beginning of World War II. The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact. The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty. The invasion is also known in Poland as the September campaign or 1939 defensive war and known in Germany as the Poland campaign.