Held:
1. In establishing loss of citizenship, the Government must prove an intent to surrender United States citizenship, not just the voluntary commission of an expatriating act such as swearing allegiance to a foreign nation. Congress does not have any general power to take away an American citizen's citizenship without his "assent," which means an intent to relinquish citizenship, whether the intent is expressed in words or is found as a fair inference from his conduct. The expatriating acts specified in § 349(a) cannot be treated as conclusive evidence of the indispensable voluntary assent of the citizen.
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2. However, the Constitution permits Congress to prescribe the standard of proof in expatriation proceedings.
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3. Nor is the presumption of voluntariness provided in § 349(c) constitutionally infirm. While the statute provides that any of the statutory expatriating acts, if proved, is presumed to have been committed voluntarily, it does not also direct a presumption that the act has been performed with the intent to relinquish United States citizenship, which matter remains the burden of the party claiming expatriation to prove by a preponderance of the evidence.
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