German–Polish relations have a long and complicated history.
From the 10th century on, the Kingdom of Poland had relations with the Holy Roman Empire, which were however overshadowed by the Polish-Teutonic wars, as a result of which, Prussia became a fief of the Kingdom of Poland. Prussia retained a certain level of autonomy under Polish rule. Later, the Kingdom of Prussia rose and eventually became one of the partitioners of Poland.
The Aftermath of World War II was the beginning of a new era. It was defined by the decline of the old great powers and the rise of two superpowers; the Soviet Union (USSR) and the United States of America (US) creating a bipolar world. Temporarily allied during World War II, the US and the USSR became competitors on the world stage and engaged in what became known as the Cold War, so called because it never boiled over into open war between the two powers but was focused on espionage, political subversion and proxy wars. Western Europe and Japan was rebuilt through the American Marshall Plan whereas Eastern Europe was in the Soviet sphere of influence and was excluded from the Marshall Plan. The world was divided into an US-led Western Bloc and a Soviet-led Eastern Bloc with some nations trying to stay out of the Cold War through the Non-Aligned Movement. The Cold War also saw a nuclear arms race between the two superpowers, part of the reason that the Cold War never became a "hot" war was that the USSR and the US had nuclear deterrents against each other, leading to a MAD situation.
As a consequence of the war, the Allies created the United Nations, a new global organization for international cooperation and diplomacy. The United Nations agreed to outlaw wars of aggression in an attempt to avoid a third world war. The devastated great powers of Western Europe formed the European Coal and Steel Community (that later evolved into the European Union) in an attempt to avoid another war by economic cooperation and a common market for important natural resources.