Being with your partner for more than a year suggests that your 'affair' is actually progressing to a relationship with some meaning. Affairs are not ideal as people can get emotionally hurt in the process. However, life's journey takes various courses and these things can happen, as it is in our nature to let our feelings for others guide us or let us 'follow our hearts' .
Your partner does appear to have endured a tremendous amount of turmoil in his marriage and he would have found the situation challenging - with her - and coping with the children as well. Her being an alcoholic is an issue in itself and there is the unfortunate side effect regarding the children as well. It must be difficult for them being in contact with the social workers, especially as they get a little older and begin to understand - or at least make assumptions as to why the social services are involved in their care.
Your partner appears to feel very happy in your company and it looks like he's very certain about committing himself to you. You've made an important, life changing decision by leaving your husband and moving into your new home and living your separate, independent life away from him.
The problem for you here, is the many people that are at present surrounding your life who are experiencing different emotional issues.
Firstly, there is your husband, who will have to deal with the end of your marriage. Then there is your daughter, who is very young but now maturing enough to experience the the emotional effects on both of you when separating and her own feelings about you not being a couple anymore.
Your new partner has come out of a very long, traumatic relationship with his wife which would very unlikely have been a straight-forward separation - especially if she was a seriously controlling, abusive bully. There would also be the issue of him 'suffering in silence' throughout the relationship as our society usually sees marital/relationship abuse as being the cause of distress for the girl/woman. Society in general 'has a vision' of domestic abuse which involves the man attacking the woman either mentally/physically. Women are generally the victims of domestic abuse.....However, so are a number of men. We just don't associate it with men being the victims.
There is also the concerning situation regarding his wife's children - presumably he is their father or had at least taken on the parental role throughout the marriage. This is going to be a worry for him even though he has left his wife and ended the marriage. How will she cope with the children?.....This could be of concern. If it is possible, helping him to maintain at least a 'peaceful relationship that is on talking terms' may help when dealing with issues regarding their children. Part of this process will require her to seek help for her drinking. This could be a possible burden lifted from your new partner's mind.
Moving into a new home is stressful in itself for you. There is also the upheaval for your daughter as well. This will be a big change for her, having to accept the fact that you and her father will not be together anymore. It may help her to deal with the situation by explaining to her why it has happened in way that's not to complicated but just enough to help her to 'feel like an adult'. Your husband is still her dad......This will help your new partner by reassuring him that your daughter and yourself are not 'in conflict' and will be one less worry for him.
Your partner will be apprehensive about meeting your daughter for the first time because if there is any conflict between him and your daughter, a strain is placed upon your relationship. At present that relationship appears to be progressing well for you both. There is the issue of them both 'getting along' together and reassurance may be needed from you that this 'new family' will not replace the previous one's. This can be like a new, 'extended family'.
Your partner has witnessed more horror/tragedy than many of us experience in a lifetime. People that work in a profession such as the army/social services/caring/police/nursing........ deal with other people's crisis/suffering and can experience a form of 'transferal' of emotions radiating from the people they are trying to help. In a way, your partner is 'taking his work home' with him. His job is not your usual 'nine to five' where he can 'just switch off'. The anniversary of his friend's death in service appears to have hit him hard and would certainly be a 'trigger' for his negative emotional state.
Will he carry on serving in the military?.....Whatever he chooses, he will need full therapeutic treatment for his mental well-being, due to his particular situation regarding military service and being in a 'battlefield environment'. Unfortunately, the mental/physical side-effects of military service seem to be 'pushed aside' or 'forgotten' in ordinary, everyday life - only brought to society's attention through 'blockbuster movies' or televised charity/celebrations. Even then, life still goes on for many, struggling with the realities of everyday life and no-one really giving 'a second thought'.
Your partner's mental health is an issue for the appropriate medical practitioner/counsellor....However, there are issues going on in your lives that could be addressed to help with his journey to recovery.
Again.....How is his relationship with the children from the previous marriage? Would it be possible for you both to communicate with his former wife regarding the children being able to visit him? The children being under the care of social services will be a worry so keeping a good relationship with them may be a good starting point for him.
Again......Meeting your daughter for the first time will be another issue for your new partner to deal with. Your daughter will not be happy about your own marriage break-up and will - of course - take the more positive view or 'side' with her dad. Perhaps taking small steps to encourage trust in each other will help your partner and daughter build a relationship. Activities/days out involving the family being together may help to encourage this.
Much of your partner's recovery really depends on all the situations around him that are going on in your lives and how you can work together to create a balance for all. Him being a soldier and feeling the effects of his work is something that he needs to work over during his therapy sessions. Your role in helping him will be to lend an emotional helping hand and mainly offering reassurance when dealing with the situations at home.