Trade in War

I for example do not understand how it can be possible that Ukraine transports Russian gas on its pipeline network. Not sure if that's still the case though.

They stopped on January 1st 2025 when the contract with Gazprom signed in 2019 expired, costing Gazprom / Russia an estimated $5bn / year: https://edition.cnn.com/2025/01/01/business/ukraine-russia-g...

It's a complicated one, but legally it's a civil contract; if the Ukrainian government decided to stop the gas flowing, both Gazprom and all the companies "downstream" would be in their rights to sue for breach of contract and/or causing gas shortages, costing the Ukrainian government billions.

And you could wonder why they signed the contract anyway given Russia invaded/annexed Crimea 5 years prior, but, it's a lot of money, and at the time it was still considered a civil contract I presume.

I am completely naive,l as I don't understand much in contracts, but wouldn't war effectively nullify those contracts? I mean, if a large proportion of your adversary's economy hangs by a simple piece of paper, I'd expect one to suggest them to go and wipe themselves with it.

I don’t think you are naive - it’s counter-intuitive. The political context is important: Ukraine is incentivised to portraying itself as a country that respects international law and norms. The fact of life is that this includes respecting civil contracts made in good faith. This moral high ground has a cost.