[mrsaint]

There are exceptions to the rule (for reference see https://en.m.wikipedia.org/wiki/Multiple_citizenship). Generally, a lot depends on whether you acquired citizenship by descent or birth, or acquired it by naturalization. Also, there are agreements between countries that you must consider.

For example, had Croatia not entered the EU a couple of years ago, it would not have been possible most likely to aquire citizenship for our children without giving up German citizenship. That is because Germany permits multiple citizenship only if you obtained another citizenship by birth and/or the other citizenship is either one of an EU member country or Swiss citizenship. Some with Croatia, which generally allows citizens by descent to have multiple citizenship, but foreigners wanting to naturalize there must renounce their old citizenship (with exceptions, of course). Switzerland on the other hand has no such rules regarding multiple citizenship.

It's difficult to not inform the countries of your multiple citizenship unless you don't apply for passports. With each passport renewal you must also state your other citizenships if applicable.

Anecdote: Shortly after the first child was born, we wanted to visit our relatives in the US. Our child only had a German passport, which we thought would be OK to travel with (because, why not?). Well, as it turned out, once we arrived at US immigration, we were in big trouble. Because our child was automatically a US citizen by descent, she was also required to have a US passport to travel from abroad to the US. This is because formally the US does not acknowledge any of your other citizenships as long as you are US citizen.

[tharkun__]

Not entirely correct. With a Beibehaltungsgenehmigung you can also obtain other citizenships than what you mentioned and it doesn't need to be by birth. They do make you answer all sorts of questions though to give you that certificate.

Source: we went through it.

We also had a similar-ish experience passport wise while the kids had non-German passports only while we only had a German one. We held up the line of cars waiting to get on the ferry to England (going from the Netherlands) for quite some time while they asked and checked why some Germans were driving Canadian kids to England: o see the Scottish Highlands, duh :)

[fakedang]

What GP meant was that USA treated the child as a US citizen by descent, in spite of the lack of a passport, which is why immigration mandated that the child should have been issued a US passport before traveling to the US. It's natural for a lot of countries that allow dual citizenship to mandate that you travel to said country with the passport of that place.

[tharkun__]

I completely understand and the two paragraphs in my comment are two separate things. One comments on the fact that they said you can only have another citizenship by birth if you want to keep the German one (false) and the other one reminded me of another sort of funny passport and different/dual citizenship situation from my life, which is why I said "similar-ish". Emphasis on "ish".

[tvanantwerp]

My wife is from Hong Kong and we've both got US citizenship. To get HK residency as well as US citizenship, our kids would have to be born in HK. We don't actively live in HK, and your anecdotes are making me wonder how feasible this would actually be within the time frame of maternity leave.

[mrsaint]

The CBP immigration officer made an exception under the premise that we would expediently apply for the US passport at a local agency during our visit. We were grateful, because of course he could also have sent us back home...