Rashi (Bereishis 1:1) quotes Rabbi Yitzchok – What was the reason that the Torah started with Bereishis and not with the Redemption from Egypt? Rabbi Yitzchok answers that it was to establish that G‑d created the world and thus He can give the land to whomever He wants. Otherwise the goyim are going to claim that the Jews are robbers by conquering the land of Israel from the Seven Nations.
Chasam Sofer (Derasha to Simchas Torah page 57b #11) asks why did the goyim object that the Jews took the land away from the Seven Nations by military conquest. After all isn't this the normal way all people take land away from it current inhabitants. So why should the Jews be considered thieves more than any other nation? An explanation is that all the ancient people believed that each people had a spiritual representative in Heaven and who provided them with a homeland. The ancient people believed that G-d was just one of the national spiritual representatives and He was the representative of the Jews. They also firmly believed that G-d did not work in the normal natural manner but rather everything that He did was through miracles in a supernatural way. Consequently when they saw that the Jews were conquering the Land of Israel in a natural manner – rather than through miracles – they accused the Jews of unlawful seizure of the land and said that they were no more than thieves. That is because they knew that G-d only worked through miracles. It was to counter this mistaken understanding that the Torah begins with the story of G-d creating the world. This showed that G-d also works in a natural manner. If the Torah started with the Redemption of Egypt, that would serve to reinforce the mistaken view that G‑d only works through miracles. Therefore now that the Torah begins with Bereishis, the Jews were able to explain that they were not thieves. They conquered the Land of Israel in a normal manner through war, because G‑d wanted a natural conquest of the land rather than a miraculous conquest. However in truth this requirement of a natural conquest was not arbitrary but was the result of the sin of the Golden Calf and the breaking of the Tablets as is well known.
So in fact there were two reasons that the Torah started with Bereishis in order to provide an answer to the Nations of the World. The first was because the Redemption from Egypt was totally miraculous so the Nations of the World mistakenly thought that all of G-d's actions were only through supernatural miracles. The second was that the breaking of the Tablets required that the conquest of land be done in a nature manner through war. [...]