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 (approximately 20 divisions still retaining the ability to co-operate) 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 plan was a default plan in case it was impossible to defend the Polish borders, and it assumed that the Polish forces would be able to retreat to the area, organise a successful defence until the winter and hold out until the promised French offensive on the Western Front had started. Rydz-Śmigły predicted that the hills, valleys, swamps and the rivers Stryj and Dniester would provide natural lines of defence against the German advance. The area was also home to many ammunition dumps that were prepared for the third wave of Polish troops, and it was linked by transport to the Romanian port of Constanța, which could be used to resupply the Polish troops. The plan is one of the reasons that the Polish–Romanian alliance was not activated by Poland. Poland and Romania had been allied since 1921, and the defensive pact was still valid by 1939. However, the Polish government decided that it would be much more helpful to have a safe haven in Romania and the safe port of Constanța that could accept accept as many Allied merchant ships as would be required to keep Poland fighting. Most of the Polish Navy and merchant marine had been evacuated prior to 1 September (see Peking Plan) and would operate from French and British ports and deliver the supplies through Romania. In adherence with the secret protocol of the Molotov–Ribbentrop Pact, which provided for the partition of Poland between Nazi Germany and the Soviet Union, the Soviets invaded Poland from the east during the early hours of 17 September and violated their non-aggression pact with Poland. However, the French, despite their promises, had not begun any significant offensive against Germany, which made it impossible for the Polish Army to hold