Prompted by a query from an American monarchist, I suppose this is as good a place as any for a list of American women who have married into royal families. (Men of course generally cannot obtain royal titles by marriage, unless they marry a female sovereign; no American man has ever done this.)
Elizabeth Patterson (1785-1879) married Napoleon's brother Jerome Bonaparte (1784-1860), later King of Westphalia, in 1803; the marriage was annulled in 1805, but they had a son who founded an American line of Bonapartes that flourished (with some distinction) until 1945.
Alice Heine (1858-1925) married Prince Albert I of Monaco (1848-1922) in 1889; they separated in 1902 but never divorced.
None May "Nancy" Stewart Worthington Leeds (1878-1923), a wealthy widow, married Prince Christopher of Greece (1888-1940), youngest son of King George I, in 1920; she was granted the title "Princess Anastasia of Greece." Her son from her first marriage, William B Leeds, then married into royalty as well, wedding his new stepfather's niece .
Wallis Warfield Simpson (1896-1986) married the former King Edward VIII (1894-1972), who had famously abdicated the British throne for her, in 1937, becoming the Duchess of Windsor.
Grace Kelly (1929-1982) married Albert I's great-grandson Prince Rainier III of Monaco (1923-2005) in 1956.
Hope Cooke (b 1940) married (1923-1982), the last King of Sikkim (1963-75) in 1963. Sikkim was annexed by India in 1975 and they divorced in 1980.
Lisa Halaby (b 1951) married King Hussein of Jordan (1935-1999) in 1978, becoming Queen Noor.
Marie-Chantal Miller (b 1968) married Crown Prince Pavlos of Greece (b 1967) in 1995.