Quoting Gaetano Salvemini’s Prelude to World War II, pages 143–4:

As for France's Eastern Allies, the hue and cry stirred up by the Four‐Power Pact had disastrous consequences in Poland. The Polish colonels, headed by Piłsudski, grew indignant with the French Government, which had accepted the principle of the Four‐Power Pact instead of a Five‐Power Pact that would have included Poland. The Polish Colonels were unable to grasp that the French Government, while apparently accepting the Pact, was out to destroy it. Had the French asked for the inclusion of Poland, they would not have been able to scuttle the Pact.

Megalomania and “patriotic” [mindlessness] were not exclusively Mussolinian diseases. As a result, there came into being the pact of friendship and non‐aggression of January 26, 1934, between Hitler and the Polish dictator, Piłsudski. The world witnessed the “extraordinary spectacle of the Polish mouse deliberately putting his head into the German cat's mouth at the moment when this redoubtable mouser was patently growing a new crop of teeth”.¹

David M. Crowe’s The Baltic States and the Great Powers: Foreign Relations, 1938–1940, chapter 1:

One of the victims of the Kremlin's efforts to find ways to gain more international security was its relationship with Berlin, particularly after the release of the Hugenberg memorandum at the World Economic Conference on June 16, 1933. In it, the conservative head of the German Nationalist party demanded, among other things, the “continued struggle against the Untermensch [inferiors] for colonies and living space for Germany and an end to existing conditions in Russia.”

Although Hugenberg released the memorandum without Hitler’s approval, it seriously undermined the Reich’s already shaky ties with the Kremlin. These ties removedd further as [the Fascists] sought closer relations with Poland, which under its new foreign minister, Józef Beck, was quite receptive to these overtures. Using the [Fascist] Volkstag victory in Danzig on May 28, 1933, as the backdrop, [the Fascists] encouraged the Danzig government to try to use diplomatic means to resolve its problems with Poland.

This action set the stage for closer ties between Warsaw and Berlin, which would “weaken the French alliances and the League system.” [Berlin’s] decision to withdraw from the League and the World Disarmament Conference on October 14, 1933, provided further impetus for this move, which resulted in the conclusion of a German–Polish nonaggression pact on January 26, 1934.

You can read about the pact’s details in Stephanie Salzmann’s Great Britain, Germany and the Soviet Union: Rapallo and After, 1922–1934, chapter 11. While the entire chapter is worth reading, for the sake of brevity I’ll simply quote what I found most striking from pages 160–1:

The Soviet government also desperately tried to improve relations with Poland and sounded out the possibility of joint preparations against a [Fascist] attack. During his visit to Poland from 6 to 22 July 1933, Radek offered the Warsaw government a Soviet guarantee for the German–Polish border.¹⁸ At the same time Moscow courted France, yet with little response until Paris was moved into action by Germany’s withdrawal from the League of Nations.

Warsaw resisted the Soviet courtship. Instead, and to its surprise, the Foreign Office learnt of a serious German–Polish rapprochement. On 21 November, William Tyrrell, the British ambassador, wired from Paris that definite negotiations between Berlin and Warsaw to settle outstanding questions could begin at any moment.

According to Tyrrell, the ball had been put in motion by Hitler’s declaration that he was willing to give assurances to fill the gap in Polish security caused by Germany’s withdrawal from the disarmament conference. So far neither disarmament nor territorial questions, nor even a non‐aggression pact had been raised but could be discussed at any time.¹⁹

(Emphasis added in all cases.)

The collaboration between German and Polish anticommunists is not a trivial footnote worth overlooking, as some nationalists have claimed. It manifested itself in several important ways, such as letting Fascists spread their propaganda and promote their films in Poland. While it would be an exaggeration to classify pre‐1939 Poland as fascist, it was in no meaningful way antifascist either.

Hopefully this is so obvious that it does not need to be said, but in case that it is not I’ll go ahead anyway: no, the collaboration between German and Polish anticommunists does not justify the Fascist invasion and destruction of Poland. Lower‐class Poles cannot be held responsible for their régime’s failures and should not have to suffer because of them—and I have reported on their suffering before as it is something that mainstream education rarely discusses. Simply put, the Polish collaborators with the Fascists were, in principle, no different from the Mesoamerican collaborators with the European colonists: just because some natives worked for the enemy doesn’t mean that the colonization of Mesoamerica was deserved.

Further reading: January 26, 1934: The Polish–German declaration of non‐violence