1939 on the River Bug
Just the opposite, Stalin doublecrossed Hitler. Both agreed to simultaneously invade Poland as agreed upon in Moscow in 1939. When the appointed time came Stalin held back his army. Germany was forced to take on Poland by itself, weakening its position and drawing the ire of the Western allies. Stalin was very clever and Hitler a bit naive. The attack on The USSR in 1941 by Germany may have appeared suicidal but in fact Hitler really had no choice. He was unable to knock out England in 1940, Germany grew weaker fighting England and knew inevitably the USA would come to their aid. He knew he could not trust Stalin again and before he could build up the Red Army into a powerful threat to attack Germany, Hitler preemptively attacked. It was a long shot gamble, and it failed. In reality the war was already decided in 1939 with Stalin's doublecross of Hitler.
Poland .
If by Danzly you mean "Danzig" then its Poland
The importance of the Soviet-German Nonaggression Pact was to give half of Poland to Stalin. With Stalin as a co- conspirator of the German battle to dominate Europe, Hitler was able to stage a two front war in 1939.
Danzig, now called Gdansk, is in Poland. The Polish Corridor separated Germany from East Prussia.
Hitler wanted Danzig, which would give him access to the Baltic Sea through Poland. Hitler wanted to build a rail system through Poland into Germany, but Poland refused. Upon Polish refusal, Hitler looked toward Russia for an alliance because if Germany invaded Poland, Hitler did not want a two front war. On August 23, 1939 the Nazi-Soviet Pact was signed which was a complete shock to everyone. No one thought Hitler who was anti-Communist would forge and alliance with Communist Russia. The Nazi-Soviet Pact was essentially a non-aggression pact between the two countries.
Because after WW1, the Danzig Corridor was given to Poland, separating east Prussia from the rest of Germany. The Bolsheviks intially considered allowing German access to their cut off territory. The USA (indirectly) and UK offered assurances to Poland, who set about murdering Germans, and refusing access to the territory. Hitler invaded, and the UK declared war.
The excuse he gave to the German people was to give them "living space". In other words, he expected to replace Polish people with German people.
After war 1 Germany had to give land and that land they were banned from.The war of world war2 started by Germany in attack of Poland. Britain and France declared war on Germany after Hitler had refused to abort his invasion of Poland.
Actually, I would place this under a discussion, but I would say he was among the top 3. I would say the top three were Stalin, Hitler, and Pol Pot (Cambodia). I can give details about them all if you would like.
Hitler before he invaded Poland in 1939 made a pact with the USSR to give the USSR half of Poland, many people don't know that. That pact disintegrated when Hitler invaded the USSR in 1942. But his other allies were Japan and Italy. Japan was his greatest ally.
Hitler aimed to conquer most of Europe during his time in power, in order to give the so-called "master race" room to grow and spread out. Two of the countries he annexed in pursuit of this goal were France and Poland.