[readhn]

"USSR attacked Poland on 17-th of September 1939 (co-starting the 2WW, together with Germany)."

German army was to complete the occupation of the Sudetenland (part of Czechoslovakia) by October 10 (1938), and an international commission would decide the future of other disputed areas. Czechoslovakia was informed by Britain and France that it could either resist Germany alone or submit to the prescribed annexations. The Czechoslovak government chose to submit.

The Soviet Union also had a treaty with Czechoslovakia, and it indicated willingness to cooperate with France and Great Britain if they decided to come to Czechoslovakia’s defense, but the Soviet Union and its potential services were ignored throughout the crisis.

https://www.britannica.com/event/Munich-Agreement

[petre]

They ended up with the USSR coming to their "defense" anyhow, just like they also did later, in 1965.