Under the new military retirement system just approved by Congress, service members will have to make a choice few of them are prepared for: Whether to take traditional pension checks or opt for a lump sum that’s worth up to half of what the pension benefit would bring.

Military members are known for devotion to duty. But service members in many areas depend not just on their military pay but on food stamps and other outside sources of income to make ends meet.

Now they have to decide, at retirement, whether to take some of the money now or a much larger amount later—a choice that was opposed by both the Defense Department and many veterans’ advocates.