What if they start taking bigger and bigger portions of Russia?

If Ukraine takes over Russia, we will be able to invade China as well.

I think your confused. Ukraine has not invaded Russia. Russia has invaded Ukraine. There is an entire topic about it here: https://forums.d2jsp.org/topic.php?t=92094408&f=119

but to answer your question, IF Ukraine invaded Russia it would make them the aggressor. in such a scenario, IF Ukraine were to keep invading, and Russia were losing, then obviously Russia would probably resort to their final deterrent, which is nuclear.

Nah. Taking territories from the invader is legit and has happened through all of history.

The lesson is don't start shit you can't finish

Based on that logic, the Soviets were the aggressor in WW2 once they attacked Germany.

that is true.

I think you need to reevaluate your stance. What happens if Russia starts bombing Ukrainian positions from Kursk and the only way to stop it is to move 10 km into Russian territory. Is Ukraine the aggressor there?

History according to the victor. never found the body. Merica' gave him & his scientists sanctuary - plum island ny,
among much of the nazi designed bio weapons they are currently using on the american public on a daily basis.

When should we start ceding territory to the Muslim countries we occupied for decades?

territory belongs to the entity which has the ability to hold and administrate it

right of conquest

no land by definition can "belong" to anyone outside of this definition.

USA is conquered land, Canada is conquered land, absolutely all land was conquered by an entity at one point

Therefore if Ukraine pushes back and advances into Russia, then Ukraine just became a bigger country