Why Would States Risk Being Dragged into Another War by Joining

Why would states risk being dragged into another war by joining an alliance?

A) Since they are in an alliance together,states can trust their allies not to risk a war.
B) A state can easily leave an alliance if its partners needlessly go to war.
C) Allies can control what the alliance does,and the alliance will not be dragged into a war without the whole alliance's approval.
D) States can clearly signal whether they will support their allies if a war begins and will be able to deter enemies from attacking.
E) Alliances make war more likely and states can often gain from a successful military intervention.