German–Polish Non-Aggression Pact
The German–Polish Non-Aggression Pact (German: Deutsch-polnischer Nichtangriffspakt; Polish: Polsko-niemiecki pakt o nieagresji) was an international treaty between Nazi Germany and the Second Polish Republic, signed on January 26, 1934. Both countries pledged to resolve their problems by bilateral negotiations and to forgo armed conflict for a period of ten years. It effectively normalized relations between Poland and Germany, which were previously strained by border disputes arising from the territorial settlement in the Treaty of Versailles. Germany effectively recognized Poland's borders and moved to end an economically damaging customs war between the two countries that had taken place over the previous decade. Before 1933 Poland had worried that some sort of alliance would take place between Germany and the Soviet Union, to the detriment of Poland. Therefore, Poland had a military alliance with France. The Nazis and the Communists were bitter enemies of each other, so when Hitler came to power in 1933 the likelihood of hostile alliance seemed remote.
One of the most noted of Józef Piłsudski's foreign policies was his rumored proposal to France to declare war on Germany after Adolf Hitler had come to power, in January 1933. Some historians speculate that Piłsudski may have sounded out France regarding the possibility of joint military action against Germany, which had been openly rearming in violation of the Versailles Treaty. France's refusal might have been one of the reasons that Poland signed the German–Polish Non-Aggression Pact. However, the argument that the German-Polish non-aggression pact had been forced on Piłsudski by French refusal to wage a "preventive war" has been disputed by historians, who point out that there is no evidence in French or Polish diplomatic archives that such a proposal was ever advanced. They state that when in late October 1933, rumors of a Polish "preventive-war" proposal were reported in Paris, their source was the Polish Embassy, which had informed French reporters that Poland had proposed a "preventive war" to France and Belgium, but Poland and Germany had already been secretly negotiating their Pact. It has been argued that Piłsudski had the Polish Embassy start rumors about a "preventive war" to pressure the Germans, who were demanding that Poland abrogate its 1921 Franco-Polish alliance. The Pact would specifically exclude that alliance.
It has been said that Piłsudski's reason for seeking the Pact with Germany was his concern over France's Maginot Line. Until 1929, French plans had called for a French offensive into the North German Plain, in conjunction with offensives from Poland and Czechoslovakia. The construction of the Maginot Line, which began in 1929, indicated the French Army's preference for a strictly defensive stance, leaving, consequently, its eastern allies on their own. (That is exactly what happened in 1939 with the Phoney War.) From Piłsudski's viewpoint, in light of France's military plans, a non-aggression pact with Germany would be the best choice for Poland.
Piłsudski used Hitler's rise to power and international isolation of Germany's new regime as an opportunity to reduce the risk that Poland would become the first victim of German aggression or of a Great Power deal (especially the Four Power Pact). Germany's new rulers seemed to depart from the traditionally Prussian anti-Polish orientation. Piłsudski regarded the new chancellor as less dangerous than his immediate predecessors, such as Gustav Stresemann, and he saw the Soviet Union as the greater threat and even opposed French and Czechoslovak efforts to include the Soviet Union in a common front against Germany.
The Poles insisted on stating that it did not nullify any previous international agreements, in particular the Franco-Polish Military Alliance. Nevertheless, by easing Poland's disputes with Germany bilaterally, the treaty weakened France's diplomatic position against Germany.
To allay any fears of a war against the Soviet Union, on May 5, 1934, Poland renewed the Soviet–Polish Non-Aggression Pact, which had been first signed on July 25, 1932. It was extended until December 31, 1945, ignoring Hitler’s repeated suggestion to form a German-Polish alliance against the Soviets. 
Poland was able to maintain friendly relations with Germany for the next five years but also with France and Britain. However, it may have also led to foreign policy inattentiveness regarding the activities of the crumbling League of Nations and ignoring the collective security schemes proposed by French and Czechoslovakia in the early 1930s.
The Pact, soon followed by a trade agreement with Germany, is said to have granted Germany a settled eastern border and allowed Hitler time for rearmament. Five years later, Germany and the Soviet Union invaded Poland. Piłsudski distrusted German intentions on the whole but perceived Hitler's origins as an Austrian rather than a Prussian as a mitigating factor and stated that Hitler should stay in power as long as possible. 
The Pact has been seen as an instance of political weakness brought on by Piłsudski's illness, and it was likened to the interwar lack of leadership displayed by Neville Chamberlain and Paul von Hindenburg.
German policy changed drastically in late 1938, after the annexation of Sudetenland sealed the fate of Czechoslovakia, and Poland became Hitler's next target. In October 1938, German Foreign Minister Joachim Ribbentrop presented Poland with the proposition of renewing the Pact in exchange for allowing the Free City of Danzig to be annexed by Germany and the construction of an extraterritorial motorway and railway through the Polish Corridor, with Germany accepting Poland's postwar borders. Poland refused. As a consequence, Hitler rescinded the Pact unilaterally on April 28, 1939, during an address before the Reichstag, as Germany renewed its territorial claims in Poland. After another few months of rising tension, and following the execution of the Molotov–Ribbentrop Pact between Germany and the Soviet Union, which contained a secret protocol by which Hitler and Stalin agreed to divide Poland between them, Germany invaded Poland on September 1, 1939, initiating World War II.