That is not true, since Hitler had wanted war way earlier than the Generalstab (General Staff), as he pointed out in 1936 that Germany had to be ready for war within four years. If it had been up to him, he would have gone to war over Czechoslovakia and consequently Europe in May 1938, before the Munich Agreement over the Sudetenland. Hitler had said then to his generals the east will be dealt with first, then the army would get a few years before engaging in the west. Back than, Chief of Staff, General Halder, had warned in a memorandum that this would mean „finis Germania“, the end of the of Germany because of the incomplete rearmament and Britains and especially Frances stance.

Two main factors contributed to the 1939 decision - the concept of Lebensraum (habitat or living space) for Germans in the east, meaning Poland, the USSR, Ukraine etc., and the Soviet Union as the embodiment of Communism and Bolshevism per se. Since there was no direct border for an invasion, Poland, bluntly speaking, was simply in the way: It is noteworthy, that Hitler tried to win over Poland for an invasion of the USSR for a few years. There was even some mutual cultural exchange program in place. Since all efforts did not bring the results Berlin wanted, Ribbentrop handed an 8-Points Program to the Polish government in October 1938. Among the demands was the reunification of Danzig with the Reich, an extraterritorial road through the Polish corridor from Germany to Danzig and Polish membership on the Anti-KomIntern-Pakt, the German led anti-Soviet pact with Italy and Japan. In compensation, Germany offered a longer non-Aggression-Treaty over 25 years, recognition of German-Polish border which had not been done. Basically, that would have meant that Poland would lose, almost if not all autonomy and get more or less under German rule. It is understandable, that Warsaw rejected this program as unacceptable in March 1939.

After Britain and France did not engage Germany after the occupation of the rest of Czechoslovakia and turning Slovakia into a Geman satellite state, the decision to go to war with drew closer. Appeasement meant weakness to Hitler, and the behaviour of Britain and France just made him bolder. In April 1939, a study by the German navy concluded, Europe from the German western border to the Ural should be under the Axis hegemony. The previous idea had been that if Poland joined the attack, that would be as welcome as if Poland stayed neutral. For the case of Poland being hostile, the German generals considered that to be just as fine for the same reason an attack of Poland would be also an option: The shape the Polish army was in, so the same month, the German army was ordered to prepare a strategy for an invasion of Poland. Also at the end of the same month, the German-Polish non-Aggression treaty (signed for,a duration until 1944) was cancelled and in May 1939 Hitler decided that „Poland will not be spared“. However, diplomatically, the Reich was standing alone. It was the Hitler-Stalin-Pact that secured Germany to the East and potentially to the West, since Britain and France had guaranteed to Warsaw they would declare war on any attacker of Poland. Hitler assumed they would not do that now, since it would have meant war against the Axis and the Soviet Union as unlikely but factual allies. So it was the Summer of 1939 that convinced Hitler to commence as planned. However, the initial invasion in August had to be cancelled abruptly even though SS Secret Service units had already begun staging false flag operations along the border the same day, because Mussolini withdrew Italian support. While making more unacceptable demands in London and Warsaw and running a propaganda campaign, Hitler ordered the attack to begin on September 1st. However, unlike in Sudetenland, Rhineland, Czechoslovakia and Slovakia, the British and French did act upon Hitler crossing the red line again.

Sources:

Hermann Graml: Europas Weg in den Krieg. Hitler und die Mächte 1939. Oldenbourg Verlag, München 1990.

Klaus Hildebrand (ed): 1939. An der Schwelle zum Weltkrieg, Berlin 1990.

Rolf-Dieter Müller: Der Feind steht im Osten. Hitlers geheime Pläne für einen Krieg gegen die Sowjetunion im Jahr 1939, Berlin 2011.

Gerhard L. Weinberg: Hitler's foreign policy. The road to World War II, 1933–1939, New York 2005.