It's not a question of whether the two parties recognise it. It's whether everyone else recognises it.
And as for the honour system question, exactly! They didn't honour the borders and thus that land changed hands. Now will everyone else recognise Russia's new land? Or disagree/go to war (ie, WWII, no one recognised Germany's actions as being okay enough to let them do it)
There's no ruling body, and no legal ownership of land just everyone agreeing to the honour system, until they break it. Then things change (for the better or worse depending on who is doing what).
"We agree to do what the UN says" rather than, "The UN is throwing the entire Israel in prison for breaking the law"
Every once in a while something happens, and the nations affected need to suck it up, or go to war. China has mostly sucked it up with Taiwan, they're just being dicks, whereas Israel is going to war.