Power gives you rights?

Since I have never watched Avatar in its entirety(one fight scene), I can only recall that Avatar was mainly about a utopia being dictated and on the verge to being conquered. I recall humans, particularly white humans, who discovered this world and try to conquer it to make it theirs. But the people on the planet fought back with means of clever tactics. Now this theme seems to reoccur through out history, even recently. The war in Iraq perfectly describe it. Powerful and wealth people, the United States, decided that they need to make everyone else more efficient, in this case Iraq, just because we have the power too. The Spaniards conquered various parts of south America and converted everyone because they felt they were superior only because they had more power. Here in the USA the Native American's land and culture was practically destroyed by Immigrants from Europe. If the Native Americans were there first what rights do people have to come in and conquer it? Well power & wealth are extremely powerful things. For instance imagine a bully, big and tall picking on a underclassmen who is short and scrawny. Why doesn't the victim fight back? Well because if they did nothing would happen. Because the conquerors were more powerful and advance they felt they had the right to give these people wait they have and convert them all, destroying their native culture. Since this theme is not solely a human characteristic, where animals too are to conquer the weak, ants conquering termites, lions conquering hyenas, this causes the theme to be universal, so it only makes conquering the weak natural. So if something is natural does it make it right? Just because you can doesn't mean you should, yet this happens everywhere, if only one would take the time to look around.