Learn Finnish: Uhraan nappulan puolustuksen puolesta. - I sacrifice a piece for the sake of the defence.

While this lesson introduces postpositions that require the partitive case (like varten), the postposition puolesta (for the sake of / on behalf of) is a very common postposition that requires the genitive case. That is why we use the genitive puolustuksen here instead of the partitive.

Could I use the partitive postposition varten instead of puolesta in this sentence?

Yes, but the focus shifts slightly. If you say puolustusta varten (using the partitive), it means you are sacrificing a piece 'for the purpose of' building or preparing a defense. Using puolustuksen puolesta emphasizes doing it 'for the sake of' the defense or in the name of it.

Why does the object nappulan end in -n rather than being in the partitive case?

The verb uhrata (to sacrifice) takes a total object when the action is viewed as complete. Since this is an affirmative statement where one whole piece is being sacrificed, the object takes the accusative ending -n (which looks identical to the genitive). If the sentence were negative, it would require the partitive.

I recognize a similarity between puolustuksen and puolesta. Are they related?

Yes! They both come from the root word puoli (side). Puolustus means defense (taking a side), and the postposition puolesta comes from the elative case of puoli, literally meaning 'from the side of', which is how it came to mean 'on behalf of' or 'for the sake of'.