In short, your responsibilities are to not discriminate because of their service and to offer them their job back after military-related absences.
The rights of applicants and employees who serve in the uniformed military services are protected by the Uniformed Services Employment and Reemployment Act (USERRA). Under this act, it is unlawful for an employer to discriminate in hiring, reemployment, retention, promotion, pay, or any benefit of employment due to a person’s military service or intent to apply for military service.
You should allow the employee to take unpaid leave to attend deployments, scheduled drills, and annual training. When the employee returns, they should be restored to the position they would have been entitled to if they had remained continuously employed.
In addition to the protected military leave, some employers offer extra time off after deployments or provide other support to help their employees who serve in the armed forces. Offering this sort of additional support can make an organization an employer of choice for members of the military services.