Nobody is inherently entitled to the land under their feet.
Countries are formed according to what makes sense in terms of geography, religion, trade, shared culture and demographics. There have been many conquests over thousands of years, and countries regularly shift their borders around as things get redefined.
WW2 was a horrific time for the entire world and an awful lots of countries had to suck up the lasting damage it caused. But suck it up we must, because the only way is forwards, and hanging onto grudges does nothing but brings more misery.