The outbreak of the First World War created an unprecedented situation in the Polish lands. For the first time since the destruction of the old Polish-Lithuanian Commonwealth the three partitioning powers (or their successor states) were now at war with each other: on the one side Germany and Austria-Hungary (the Central Powers), and on the other Russia in alliance with France and Great Britain (the Entente). Some patriots in the previous century had considered that an independent Poland could only emerge out of a great European war. Yet there was nothing inevitable about Polish independence. Although both sides in the war were to make general appeals to their Polish populations, for neither of them was Polish independence initially a war aim. However, by the end of 1918 the core of an independent Polish state had come into being, made possible by the unexpected collapse of the three eastern empires, and by the readiness of the Poles to exploit this advantageous situation.

The outbreak of the war found the Poles bitterly divided. The anti-Russian Piłsudski was the first to act, anxious to establish a distinct Polish military presence in the war with Austrian permission. The incursion of his riflemen into the former Kingdom in August 1914 ended in a fiasco; the local population was unmoved by the appeal to rise against the Russians. As a result Piłsudski had to submit his legions to stricter Austrian control.