Complex sentence: In the first two decades of the 13th century, the (Western) Latgalians allied with German (mainly Saxon) crusaders, and their lands (Eldership of Tālava, Principality of Jersika, Principality of Koknese) as vassal states were incorporated into Livonia.
Split into simpler sentences: In the first two decades of the 13th century the (Western) Latgalians allied with German (mainly Saxon) crusaders. Their lands (Eldership of Tālava, Principality of Jersika, Principality of Koknese) as vassal states were incorporated into Livonia.