Per https://www.globalcitizensolutions.com/americans-dual-citizenship/ https://www.globalcitizensolutions.com/americans-dual-citize... "...it’s important to re

Per https://www.globalcitizensolutions.com/americans-dual-citizenship/ https://www.globalcitizensolutions.com/americans-dual-citize... "...it’s important to recognize that the USA follows a ‘master nationality’ rule. What this means is, it recognizes only the US nationality of an individual, regardless of whether that individual holds any other citizenship." So it's not that it doesn't allow it so much as it just pretends that your additional citizenship(s) don't exist. The USG will come after you for entering/leaving the US using a non-US passport though.