If you don't have your US passport, stop reading this and go get one now.

Passports are probably the best evidence of citizenship, but when people talk about getting a second passport they're really talking about acquiring citizenship in another country and becoming dual nationals. For U.S. citizens, this is not a problem. Since the decision of Afroym v Rusk in 1967, the US has recognized the right of individuals to hold citizenship in more than one country, and the Supreme Court held that Congress has no express power under the Constitution to strip a person of citizenship. Laws that stripped a person of US citizenship for things like voting in a foreign election or military service in a foreign country were nullified by this decision.

Passports really didn't come about until the turn of the 20th Century, with the advent of cheap travel for the masses. The passport is a tool of control that is ostensibly to limit the influx of foreigners into a country, but in reality it's been a governmental tool that allows a country to control its own population.

Always keep that in mind.