Poland is the recreated nation from World War I. It was given the Polish corridor so it could have access to the Baltic Sea. Both territories had been a part of Prussia, and later the German Empire, from the 1772 until 1918.
Germany
Denmark
Denmark.
Belarus is not a Baltic nation. The Baltic nations are Estonia, Latvia, and Lithuania, located in Northern Europe along the Baltic Sea. Belarus is a landlocked country in Eastern Europe.
Sweden
If your question is about geography, the answer is nothing, the two nations share a common border. The Poles are unfortunate in that their land has no natural, or defensible boundaries. So, over the centuries any invader can pretty much just roll right in, from any direction. This led to Poland's disappearance from the map as an independent nation in the 1700s. Poland reappeared when maps were redrawn as part of the Versailles Peace Conference process following WWI. In between the world wars Germany was geographically divided because the Versailles Peace Conference gave Poland the city of Danzig, on the Baltic Sea, so the newly resurrected Poland would have an ocean outlet. The city of Danzig (Gdansk today) was connected to the rest of Poland by a long, narrow strip of territory running south from Danzig to where the bulk of Poland was then. This was called the "Danzig Corridor", and part of Germany, East Prussia, was to the east of the Corridor, and separated from the rest of Germany by this corridor. You may be sure Hitler despised the Danzig Corridor.
Until Otto von Bismarck unified Germany in the 1860s creating the "Second Reich" Germany consisted of many small independently ruled nations. From 1920 until 1939 the East Prussian state of Germany was separated from the rest of Germany by the Danzig corridor of Poland, but was still politically part of Germany and not a separate nation. From 1945 until 1990 Germany was partitioned into West Germany and East Germany.
The Soviet Union had a non aggression pact with Germany & took part in the division of Poland in 1939. Also they annexed the baltic states of Latvia, Lithuania & Estonia. This lasted until the summer of 1941 when Germany invaded Russia.
Germany is its own country.
Germany was unified into one nation on January 18, 1871.
Germany is a state.