The Kingdom of Poland (Polish: Królestwo Polskie), also known informally as the Regency Kingdom of Poland (Polish: Królestwo Regencyjne), was a proposed puppet state of Germany during World War I. In reality, the German Empire planners wanted to annex around 30,000 square kilometers of the territory of former Congress Poland and carry out ethnic cleansing up to 3 million Poles and Jews to make room for German colonists in the so-called Polish Border Strip plan. The decision to promise restoration of Poland was taken by Germany in order to attempt to legitimize its military occupation amongst the Polish inhabitants, following upon German propaganda sent to the Poles in 1915 that German soldiers were arriving as liberators to free Poland from subjugation by Russia. A draft of a constitution was proposed 1917. German government used punitive threats to induce Polish landowners living in the German-occupied Baltic territories to move to the state and sell their Baltic property to Germans in exchange for moving to Poland, and efforts were made to remove Poles from Polish territories partitioned by Prussia.

Following the armistice with Germany that ended World War I, the area became part of the Second Polish Republic.

In 1914, for the purposes of securing Germany's eastern border against Russia, Theobald von Bethmann-Hollweg, the German chancellor, decided on the annexation of a strip of Polish land. In order to avoid adding the Polish population there to the German Empire, it was proposed that the population would be moved to a new Polish state further east, while the strip would be resettled with Germans.

...
Wikipedia