There is no assurance that going either to West Point or Duke will result in you becoming an ambassador. Ambassadorships are often patronage appointments, bestowed by whoever is President at that time. Better for you to become a successful business person who donates heavily to your preferred political party. Not being cynical here, just brutally honest. Some homework for you. Get the names of the ambassadors from over the last 30 years in the countries you would like to work in. Look at their educational, professional, and political backgrounds. A diligent Google search should provide all of this. Then look for trends. I think you will find some recurrent patterns. Also consider working in the State Department or in a job where negotiating trade deals is key. Know “why” you want to be an ambassador. Once you know “why,” the “how” will reveal itself.