This is the first "battle" featured in the show, in Rites of Passage. In the battle, Ragnar and Rollo, probably under Earl Haraldson's command, are seen fighting against Eastern Balts. This battle is before the story begins, before Ragnar's desire to explore the West. While under Earl Haraldson's rule, the Earldom of Kattegat (and most of the other viking earldoms and kingdoms) used to raid the Baltic Sea and fight against the Slavs, who had a tribal lifestyle and lived in small villages, always fighting each other. They were also pagans, although they had their own religion(s) and worshiped different gods than the Norsemen. They were also much poorer than the western christian peoples, like the Saxons and Franks, and even poorer than the Norsemen, as stated by Rollo later in the episode.
In the battle itself, Ragnar and Rollo are the only viking warriors, fighting alone against the last Balt tribesmen, which indicates that the battle has already ended. The two brothers finish off the last enemies, and Ragnar thanks Rollo for helping him.
Later, Ragnar has a vision. He sees Odin walking among the dead, with his spear Gungnir in hand and a large flock of crows flying around the battlefield. He also sees the Valkyries taking the slain warriors to Valhalla.