Th US also has the premise that no one in his clear state of
mind would give up the US citizenship. However, I'm not sure if
this applies to naturalized US citizens.
Do Foreign Citizenship Oaths Strip you of Previous Citizenship(US
citizenship)?
In general, no, although some countries (such as the US but not
Canada or Australia) have verbal oaths that state that all former
citizenships are relinquished, there are few (if any) modern cases
in which this has happened to dual citizens. Most citizenship oaths
are historical and have little legal power. In general, most
countries that allow dual citizenship require very specific acts
for you to relinquish your citizenship, and routine verbal oaths
delivered in a foreign country are rarely considered valid. Marc
Rich (of Clinton pardon fame) is one of the well-known cases
(Action and Deltamar v. Rich, 951 F.2d 504 (2nd Cir. 1991)). Rich
assumed an oath in Spain would remove his US citizenship. The
Spanish naturalization oath he took included an explicit
renunciation of US citizenship. Due to a variety of circumstances
the court judged that he was still a US citizen.
The US State Department's position is:
The Department has a uniform administrative standard of evidence
based on the premise that U.S. citizens intend to retain United
States citizenship when they obtain naturalization in a foreign
state, subscribe to routine declarations of allegiance to a foreign
state, or accept non-policy level employment with a foreign
government. Both the Australian and Canadian citizenship oaths do
not require giving up previous citizenships