Must we rehire returning service members? We have already hired replacement workers

Q. We have a number of employees who serve in the armed forces. Some have taken multiple leaves in recent years because they were called up for duty in Iraq or Afghanistan. At least one has been gone for years. With the drawdown of troops in Iraq, we expect that several will want to return to our company, but we have had to hire people to replace them. Are we obligated to rehire them even if we don’t have an open position?