"Matěj slíbil, že se Rusku pomstí."

Translation: Matěj promised that he would retaliate against Russia.

[iyoossaev]

Again, all dictionaries and examples suggest this only means to take revenge, not retaliate.

[VladaFu]

take revenge is accepted

[marulin3]

Proc je would a ne will ?

[VladaFu]

souslednost časová (slíbil v minulosti)

[Paul378887]

I have 2 questions about the verbs “mslít” (mslít se?) and “pomslít” (pomslít se?). 1) Is the reflexive “se” part of the verb and always used? And 2) Is “proti” simply understood and, if used, would be redundant or superfluous?

[VladaFu]

It is mstít, not mslít.

The verb is not always reflexive. The se is comes there as the accusative of the pronoun myself/oneself,"mstít sebe" or "pomstít sebe". To avenge myself, to vindicate myself.

You can also take a revenge on someone else's behalf, avenge someone else, vindicate someone else. Mstít někoho, pomstít někoho.

If you include the dative, against whom you revenge, then it only likely works as reflexive: mstít se někomu, pomstít se někomu.

No "proti" is used here, the dative case expresses that.

You can also check https://ufal.mff.cuni.cz/vallex/4.0/#/lexeme/pomst1/0 vs. https://ufal.mff.cuni.cz/vallex/4.0/#/lexeme/pomst2/0