The Romanian Bridgehead was an area in southeastern Poland that is now located in Ukraine. During the invasion of Poland in 1939 at the start of the Second World War), the Polish commander-in-chief, Marshal of Poland Edward Rydz-Śmigły, ordered all Polish troops fighting east of the Vistula to withdraw towards Lwów and then to the hills along the borders with Romania and the Soviet Union on 14 September. After the Soviets attacked on 17 September, Rydz-Śmigły ordered all units to withdraw to Romania and Hungary, but communications had become disrupted although smaller units crossed outside the major battles.

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.