Final answer:
The Treaty of Versailles is the agreement that Germany believed unfairly took Poland from them, as it created Poland and established the Polish Corridor.
Step-by-step explanation:
The agreement that Germany believed unfairly took Poland from them is the Treaty of Versailles, which was signed in 1919 at the end of World War I. One of the conditions of the treaty was the creation of Poland and the establishment of the Polish Corridor, which gave Poland access to the Baltic Sea. This decision angered Germans who felt their lands were divided and their citizens were put under foreign control. Hitler used this resentment as a justification for his desire to regain control of Poland.