If two countries both consider me a tax resident, which one gets to tax me?

When both countries claim you as a resident, an income tax treaty's residency tie-breaker rules, usually in the residence article, decide which country treats you as resident for treaty purposes. The tests apply in order: where you have a permanent home available, then your center of vital interests, meaning where your personal and economic ties are strongest, then habitual abode, and finally citizenship, with the tax authorities consulting if still unresolved. Winning the tie-breaker as a resident of the other country can let you file the U.S. return as a nonresident for treaty purposes, but the saving clause and Form 8833 disclosure complicate this, and it does not erase FBAR or Form 8938 duties for U.S. persons. This is advanced territory. Read the specific treaty's residence article and get cross-border advice before claiming a tie-breaker position.