They can certainly give it their best shot if they want to deal with the consequences. That's how the world actually works. Institutions like the UN are almost entirely powerless when it comes down to it, while eg NATO or the various militarily powerful nations are not. You can witness that with what has gone on in the invasion of Ukraine, what has the UN stopped or prevented? Nothing. The same will hold true when China invades Taiwan one day.

The countries with potent military capabilities dictate how everything goes in this world. It was true a thousand years ago, it was true a hundred years ago, and it's true today. It will always be true, for very obvious reasons.