I suppose by valid violence you mean 'permissible' violence. In other words, violence or hinsa that does not attract bad karma. The concept of violence or 'hinsa' in Jainism has to be understood from different perspectives. First, let's be clear about the meaning of hinsa. read more
For detailed description on what is permissible violence in Jainism you may refer my answer to the question ‘What is the valid violence in Jainism’. So, there is no restriction on Jains joining military. read more
According to Jainism, a monk who is careless in his activities is guilty of violence irrespective of whether a living being remains alive or dies; on the other hand, the person who is ever vigilant and careful in observing the samitis experiences no karmic bondage simply because some violence may have taken place in connection with his activities. read more