Interplanetary wars are stopped before they happen by the Onenesses of the Galaxy that have Time Pools

How is this done? 

By Altering time (without telling the barbarians it is being done). 

Most barbarians are not allowed to have time travel equipment by the Galaxy. So, since they don't calibrate which time line they are on usually when the galaxy alters time to prevent wars they don't notice.

So, because there are so many divergent cultures in different time lines on different planets and dimensions etc. Onenesses sponsored by the Galactic Government prevent contacts between planetary cultures by altering the time lines of the ships in relation to planets or dimensions they visit to prevent wars galaxy wide.

For example, Time travel equipment is allowed (very judiciously) to some elements of the United Nations now on earth to prevent specifically nuclear wars and other chemical and biological holocausts.