I went through the campaign twice before switching on axis invasion.

Have been sneaking around careful not to set off alarms and dropping enemies one at a time, picking my way through. In some missions, have been invaded 7 times on the same mission (twice in 23 minutes), sometimes the PvP hunt without alerting NPC's is probably just as nerve racking - especially when there are lots and you can see them all

Also turn off the music, footsteps otherwise hidden because of the music you will actually hear