Complex sentence: The first treaty uniting the eight members of the confederacy was the "Sempacherbrief" of 1393, concluded after victories over the Habsburgs at Sempach in 1386 and Näfels in 1388, which forbade a member from unilaterally beginning a war without the consent of the other cantons.
Split into simpler sentences: The first treaty uniting all of the then eight members of the confederacy became the "Sempacherbrief" of 1393. This treaty was concluded after the important victories over the Habsburgs at Sempach and Näfels (1386 and 1388) and declared that no member was to unilaterally begin a war without the consent of the other cantons.