The Slovak invasion of Poland occurred during Nazi Germany's invasion of Poland in September 1939. The recently created Slovak Republic joined the attack, and Field Army Bernolák contributed over 50,000 soldiers in three divisions. Since most of the Polish forces were engaged with the German armies, which were more to the north of the southern border, the Slovak invasion met only weak resistance and suffered minimal losses.

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.