Power
Eh, it depends. Which position are you in? Which level of civil service employee are you? Are you the ambassador, or on the ambassador's direct staff? Influence is probably a better word than power, although you may have some actual power within your specific organization.
Let's not forget that a foreign service officer represents the government of the United States of America. While you personally may not wield anything that can accurately be called "power," the US of A still carries a lot of weight in pretty much every corner of the world. The exact level of influence we leverage around the globe is in flux at the moment, but we still provide the lion's share of the United Nations' operating budget, we still have the world's most powerful military force, and, while it's limping a bit right now, we also created the most dominant economic machine earth has ever seen. You may not wield power, but you represent an incredibly powerful force in the world.
Heady, right?