Quoting Michael Jabara Carley’s 1939: The Alliance That Never Was and the Coming of World War II, pages 44–5:

The war scare prompted the French government to sound out Poland about its support, though the Poles had already offered numerous indications of their intent. On May 22 Bonnet called in the Polish ambassador in Paris, Juliusz Łukasiewicz, to ask what the Polish policy would be. “We’ll not move,” replied Łukasiewicz.

The Franco-Polish defense treaty included no obligation in the event of war over Czechoslovakia. If France attacked [the Third Reich] to support the Czech government, then France would be the aggressor. Not apparently overreacting to this extraordinary statement, Bonnet then inquired about the Polish attitude toward the Soviet Union, stressing the importance of Soviet support, given Polish “passiveness.”

Łukasiewicz was equally categorical: “the Poles considered the Russians to be enemies [and we] will oppose by force, if necessary, any Russian entry onto [our] territory including overflights by Russian aircraft.” Czechoslovakia, Łukasiewicz added, was unworthy of French support.²⁴

If Bonnet had any doubts that the Polish ambassador was not accurately representing his government’s views, these were quickly put to rest by Field Marshal Edward Śmigły-Rydz. He told the French ambassador in Warsaw, Léon Noël, that the Poles considered Russia, no matter who governed it, to be “Enemy № I.” “If the German remains an adversary, he is not less a European and a man of order; for Poles, the Russian is a barbarian, an Asiatic, a corrupt and poisonous element, with which any contact is perilous and any compromise, lethal.”

According to the Polish government, aggressive action by France, or the movement of Soviet troops, say even across [the Kingdom of] Romania, could prompt the Poles to side with [the Third Reich]. This would suit many Poles, reported Noël: they “dream of conquests at the expense of the USSR, exaggerating its difficulties and counting on its collapse.” France had better not force Poland to choose between [the Soviet Union] and [the Third Reich], because their choice, according to Noël, could easily be guessed.²⁵

As Daladier put it to the Soviet ambassador, “Not only can we not count on Polish support, but we have no faith that Poland will not strike [us] in the back.” Polish loyalty was in doubt even in the event of direct [Fascist] aggression against France.²⁶

Pages 68–9:

Colonel Józef Beck was the Polish foreign minister and a key subordinate of Marshal Józef Piłsudski, the Polish nationalist leader who had died in 1935. Beck began his career as a soldier during the First World War, but after the war he was increasingly chosen for diplomatic work and in 1932 he became foreign minister.

Like Piłsudski, Beck was a Polish nationalist who hoped to reestablish Poland as a great power, as it had been in the sixteenth and seventeenth centuries. Their efforts were unsuccessful, and this failure left Polish nationalists sour and quick to take offense.

Yet they tended to carry on the business of state as though Poland was a great power—dangerous conduct in the 1930s as [the Third Reich] grew stronger and more predatory. Beck leaned toward [the Third Reich] in the late 1930s, which brought Poland into conflict with the Soviet Union. Essentially the Polish government tried to ride the tiger’s back, and ultimately could not do so. If Poland then fell out with its other great neighbor, [the Soviet Union], it would be in grave danger.

[…]

In a meeting with the British ambassador on September 24, Beck said that Poland would not “tie its hands” regarding Teschen; “it did not have belligerent intentions but it could not agree that German demands being satisfied, Poland should receive nothing.”

Put another way, Beck said he did not intend to leave to [the Third Reich] the exclusive benefits of a dismemberment of Czechoslovakia. Anyway, added Beck, there was nothing to worry about because the Czech government had indicated verbally to the Polish minister in Prague that it agreed in principle to the cession of territory to Poland.

The Poles had other ways of sending their message to Paris: when the French military attache asked for information on German troop movements, his counterpart could say little in view of the French position on Teschen. If [the Third Reich] entered Czechoslovakia, this Polish officer added, Poland would take advantage of the situation to act in its own interests.

(Emphasis added.)