You don't need a real basis for a response. There's nobody here enforcing any rules.

If you wanted you could claim you have evidence, without having any. Doesn't matter.

"This better stop or there will be sanctions, and if its not within your power to stop it, tough luck" is a valid response.

Whether that's wise is another question.

Here's a recent example where Germany so far claimed that it tied the attack to Russia, but whether that's actually true is anyone guess (since there's little publicly verifiable information released so far): https://en.wikipedia.org/wiki/2026_Leipzig_Airport_drone_att...

It wouldn't be terribly surprising if Russia was behind it, but on the other hand it wouldn't be the first false flag incident of this war either.

Well yes technically countries can just attack others. In practice they need to justify those attacks to their allies and populace if they want to maintain the status quo with them. That's where "they started it, we're only defending ourselves" is very helpful.