Complex sentence: The Treaty of Salynas was signed in 1398, leaving Samogitia to the Knights, in order to secure western front while Vytautas organized a large campaign against the Golden Horde resulting in major defeat at the Battle of the Vorskla River in 1399.
Split into simpler sentences: The Treaty of Salynas was signed in 1398, leaving Samogitia to the Knights, to quiet the Duchy's western front while Vytautas was organizing a major campaign against the Golden Horde. The Horde suffered a major defeat at the Battle of the Vorskla River in 1399.