My understanding of citizenship is that it's a relatively modern invention that a person could have more than one citizenship. The oaths you mention are holdov

My understanding of citizenship is that it's a relatively modern invention that a person could have more than one citizenship. The oaths you mention are holdovers from the time when citizenship meant fealty to a specific ruler in exchange for lifelong protection and sanctuary. To have more than one citizenship would be impossible, since you can only be "supremely loyal" to a single sovereign. These days, since citizenship is more an operation of law from a nation state and less a particular privilege granted by a ruling individual, such loyalty is less required. The vast majority of countries consider someone who has multiple citizenships, including the one of the country in which they are in, to be a citizen of solely that country (e.g. if you are a dual citizen of the United States and Australia and you are standing in the United States, the U.S. only recognizes your U.S. citizenship). Some countries, like Germany, still actively try to discourage the practice, though even Germany is becoming less strident. If you are a German citizen and you naturalize in any other (non-EU or EEA country or Switzerland), you automatically lose your German citizenship by operation of law unless you asked for permission to retain it. But getting permission has become progressively easier, and Germany does not make a person who is born with German citizenship and another citizenship choose by a certain age. It is something of an interesting topic, for sure, though it's very easy to fall down the dark well of conspiracy theorists (some Germans believe you are not a Proper German Citizen unless you have the goldenrod-colored A4 form declaring you to be so) and nation state abolitionists (not saying I disagree with the latter, but it is a very short trip from there to "I am a human and also a corporate person separate from my corporeal being").