links18 said:
But aren't you making something of a legal declaration of your citizenship every time your cross an international border, including possibly signing a form attesting to your citizenship? Of course, there is generally only space for one citizenship on such forms.
I really don't think anyone in the US would care what you did as long as you didn't willfully and with intent renounce your US citizenship. Hiding your US citizenship while applying for another country's citizenship may very well be evidence of an intent to keep it. But this is pretty theoretical. The point is that simply failing to identify yourself as a US citizen to a foreign government is very unlikely to get your US citizenship pulled against your will.
The one thing that you are failing to take into consideration is that the Canadian citizen applicant is committing a potentially expatriating act (i.e., naturalizing as a Canadian citizen). This act, in and of itself, opens the possibility that the citizen
may be committing said act with the intent to relinquish citizenship. Once this potentially expatriating act is committed, the DoS may, at anytime thereafter, turn a critical eye to this individual's actions to make a determination whether there was or was not intent to relinquish citizenship; or if the individual has committed acts that are inconsistent with an intent to maintain citizenship, regardless of intent; or if the citizen committed a [bona fide] expatriating act.
Failing to declare U.S. citizenship on a signed Canadian citizenship application is no different than officially denying that one is a U.S. citizen. This is vastly different than failing to declare U.S. citizenship verbally, while not under oath. Denying U.S. citizenship out of a generalized, non specific fear is not excusable under U.S. law. U.S. immigration law only recognizes specific, dire and imminent threats as an excuse for criminal actions (perjury is a criminal action, regardless of the jurisdiction of its commission). Thus, if your plane is being hijacked, denying U.S. citizenship is excusable. Because an immigration officer sneered at you, is not.
The problem with citing cases, as you requested, is that these cases are neither published nor publicized. There is zero public sympathy for expatriates in the USA, when bad things befall them, they are seen as getting their just deserts -- American exceptionalism is almost universally embraced in America's public sphere. There is a rumoured case of a woman who had her U.S. citizenship revoked because she failed to list her U.S. citizenship on her application for a Canadian passport -- but I can provide no citation for this event.
On the other hand, there are literally hundreds of cases of U.S. born individuals who have had their U.S. citizenships revoked and been deported to countries because (i) a claim to citizenship through descent can be made by DoS on their behalf; and (ii) the U.S. is in a position to force that country to accept deportees, whether they want them or not (Jamaica being a prime example).
I will agree that the chances of having one's citizenship revoked simply because they committed perjury to a third nation by failing to declare their U.S. citizenship is remote, even extremely remote. But committing such a deed is also indicative of an individual likely to commit other deeds, which taken together, may lead DoS to find an intent to relinquish in their actions. Let's take a look at a hypothetical situation:
An individual applies for Canadian citizenship and fails to declare U.S. citizenship. After becoming a Canadian citizen, she votes in Canadian elections. She does not continue to vote in American elections. She works in Canada and files Canadian tax returns. Because she does not owe U.S. taxes, and does not own as much as US$ 10,000 in non U.S. bank accounts, nor any assets, she does not file U.S. taxes -- which is perfectly legal. She acquires a Canadian enhanced drivers licence. She allows her American passport to lapse because she does not travel. Once or twice, she has travelled to the USA on her Canadian enhanced drivers licence, because she no longer has a valid U.S. travel document. Taken as a whole, DoS personnel may conclude that this is the behaviour of someone who intended to relinquish U.S. citizenship and revoke her citizenship on that basis (without necessarily notifying her of their action). This sort of thing has happened countless times. There are many Americans who have migrated to Canada and lost their citizenship this way. Many of them were refused U.S. passports in the 1970s, 80s and 90s. And some of these ex-citizens have had their U.S. citizenships retroactively restored by DoS since 2010 (because of FATCA?), making them liable for form-crimes and potentially bankrupting penalties for failing to file U.S. tax returns.
If there is only room on the form for a single citizenship, then write very small and make sure "USA" is included.
You ask, "I am not sure why you are so fascinated with this?" I am a multinational American born citizen. The U.S. has, on several occasions, threatened to revoke my citizenship, threatened to deport me, and threatened to declare me inadmissible to the U.S. I am currently living in-exile and making the best of life in Canada (thank you Canada for taking us in!) While my life is not a worse case scenario, it is certainly not a best case scenario either. Maintaining U.S. citizenship is important to me and, as a multinational and occasional expatriate, I have made myself an expert on how to protect my status. I share my knowledge (and paranoia?) here to help others safeguard their U.S. citizenship.