At this point, in the first aliyah, the Jews are at the end of 40 years in the desert, and are situated just across the Jordan River from Israel. They have just conquered the Midianite nation, and Moshe Rabeinu's passing away is imminent. In this situation, Moshe Rabeinu begins a rebuke of the people for their misdeeds of the past 40 years. Rashi says that Moshe learned to rebuke just before passing away from Yakov avinu, who rebuked his sons right before his death. This aliyah begins the rebuke.