While Guam is a US territory, they have their own tax system so you want to claim credit on the US tax return for the taxes you paid on your Guam return. Since you were there less than a year, you may not be required to file a Guam return. Check with Welcome to GuamTax.com | The Official Web Site of the Guam
You will file Form 1116 for foreign tax credit. See Claiming the Foreign Tax Credit with Form 1116 - TurboTax
See also Individuals living or working in a U.S. Territory.
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