No, not the Federal Republic of Germany. Nazi Germany. We already won, you say? Well, they do still seem to be occupying pockets of France.
The Geneva Conventions make a clear distinctions between combatants - people who identify themselves with some form of uniform - and non-combatants, who are not in uniform and hence not part of the fight.
So if you find yourself in a restaurant with some Nazi soldiers, all you have to do is identify yourself as part of a military force, and the war can be on. Ideally, carry a cap or helmet with you, though traditionally speaking, a brightly-coloured handkerchief tied around your shoulder together with a verbal declaration of your combatant status should do as well.
While it is a bit unclear whether you are allowed to shoot the enemy soldiers at this point, as they are likely unarmed, you are definitely allowed to take them prisoners of war. You can then keep them in a prison camp (perhaps a large pit in your back yard?) until you receive confirmation of a formal surrender from a high Nazi official: say Hitler, or maybe Goebbels or Goering. Until the time where you have Hitler on your phone surrendering, you may keep them in that pit.
I envision a glorious if rather lengthy war.