If you can't claim a credit for the full amount of qualified foreign income taxes you paid or accrued in the year or you had no foreign income this year, you're allowed a carryback and/or carryover of the unused foreign income tax. You can carry back for one year and then carry forward for 10 years the unused foreign tax.
Please see Publication 514 regarding carryovers: https://www.irs.gov/pub/irs-pdf/p514.pdf
[Edit 03.18.20 | 1:47 pm]
