In 1967 and 1980, there were two Supreme Court rulings that effectively removed the earlier prohibition against dual citizenship. Here’s a summary that I found on the internet:
Afroyim v. Rusk (1967): This landmark case held that U.S. citizens cannot be involuntarily stripped of citizenship. Afroyim, who had voted in an Israeli election, was targeted under a provision of the INA for allegedly relinquishing his U.S. citizenship. The Court ruled that citizenship is a constitutional right under the 14th Amendment, and Congress could not revoke it without the individual’s consent.
Vance v. Terrazas (1980): While reaffirming Afroyim, the Court clarified that the government must prove, by a preponderance of the evidence, that a citizen intended to relinquish U.S. nationality before denaturalization could occur. Dual nationality alone is insufficient grounds to revoke U.S. citizenship.
I received Austrian citizenship a couple years ago under their law that restores citizenship to those persecuted under the Nazi regime and their descendants. I have found this citizenship useful for some travel destinations where Americans must get a visa, but Austrians do not (China, Brazil). I also breeze through passport control in EU countries and don’t need to worry about the Schengen time limits.