Short answers, pulled from the story.
The United States passed the Expatriation Act of 1868 to grant Americans the right to freely renounce their US citizenship. This legislation followed diplomatic conflicts involving British naval officers boarding American ships in 1812 and claiming sailors as subjects of Great Britain.
Jus sanguinis allows countries to grant citizenship based on descent from parents or grandparents regardless of birth location. Jus soli grants unconditional birthright citizenship to anyone born within a territory, though Australia, France, Germany, Ireland, New Zealand, South Africa, and the UK have modified these rules requiring at least one parent to be a citizen or legal permanent resident.
Five Caribbean nations including Antigua and Barbuda, Grenada, Dominica, Saint Kitts and Nevis, and Saint Lucia offer citizenship by investment programs. Vanuatu, Montenegro, Turkey, and Jordan also provide citizenship by investment options while Portugal offers a residence program with a five-year timeline before eligibility for citizenship.
Germany restricted dual citizenship until the 27th of June 2024 when new legislation allowed all cases of dual nationality without requiring renunciation of previous citizenship. Austria permits dual citizenship only for those who obtained another citizenship by birth.
Spain maintains dual citizenship treaties with Argentina, Bolivia, Chile, Colombia, Costa Rica, Dominican Republic, Ecuador, Honduras, Guatemala, Nicaragua, Paraguay, Peru, Puerto Rico, and Venezuela. Spaniards residing in these territories do not lose their rights as Spaniards if they adopt that nationality but Spanish citizenship is lost three years after acquiring foreign citizenship unless declared otherwise.