Adam3000
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The thing is u can't go back before aegon because there was no realm as we know it. U can say the children of the forest because they were United as far as we know and were all over westeros, but even they, for all we know, could have had their own little sets and factions. After them it was the first men and the first men and children fought but then made an alliance to fight the others and after beating the others, a bunch of kingdoms set up all over and the age of heroes began. So there was never a set ruler of all of westeros until aegon so legally there is no way to take it back before him.
Even the time lineage starts with him. It's all 300 bl or 300 al. Aegons landing is as significant as Christ to them lol. He was the first ruler of the whole continent. Even Robert said it "there are still those who call me usurper". He won it by rebellion and even tho he ran it for years and became a loved king, even he knew his hold was not as strong as he liked, which again is why he was so hellbent on killing Danarys and viserys.
So when Christopher Columbus 'discovered' America, then the Europeans followed and claimed it as English land thus establishing a dynasty on soil which previously had no leadership, they by right hold dominion over the land for life just because they claimed it at an arbitrary point in time?