Asalam Walikum, Are women joining the military haram? If so, why?
Alaikumassalam wa rahmatullah,
According to a fatwa issued by al-Azhar University, it is permitted for women to work in the police force and the army. There are examples of women working in the army during the time of the Prophet 
. They mainly worked as nurses but sometimes they also joined the fighting.
Women face a high risk of sexual harassment and rape in the army from what I have read, for this reason it is best if Muslim women avoid army jobs that require constant unsupervised mixing with soldiers. Secretarial, nursing and technical jobs in the army would be the best choice. It is not a clear-cut issue between halal and haram; people should use their own judgment in deciding whether the work environment is safe enough and reputable enough for a Muslim woman to work there. It also changes from woman to woman; some women are physical much stronger than others and have a dominant personality. Such women can easily avoid and counter abusive men, while physically weaker and more submissive women may not be able to do so.
Sources:
- Al-Azhar fatwa (Arabic PDF)