Parshat Devarim - Moses' Speech

Sefer Devarim contains a series of speeches by Moshe Rabbeinu. The first few chapters seem to comprise an introduction to the second, significantly longer speech. Why do we need this introduction, and why does its historical account contain major gaps, leaving out the ten plagues and the splitting of the Red Sea? The first three chapters are meant to lead to a conclusion. The ten plagues are irrelevant to this speech’s purpose, which is not to highlight miracles or wonders, but to emphasize the secret to succeeding in the future:  Bnei Yisrael need to turn to God and follow God’s law instead of believing that they themselves are the sole architects of their own success.