26 USC 2101 - Tax imposed

(a) Imposition 
Except as provided in section 2107, a tax is hereby imposed on the transfer of the taxable estate (determined as provided in section 2106) of every decedent nonresident not a citizen of the United States.
(b) Computation of tax 
The tax imposed by this section shall be the amount equal to the excess (if any) of
(1) a tentative tax computed under section 2001 (c) on the sum of
(A) the amount of the taxable estate, and
(B) the amount of the adjusted taxable gifts, over
(2) a tentative tax computed under section 2001 (c) on the amount of the adjusted taxable gifts.
(c) Adjustments for taxable gifts 

(1) Adjusted taxable gifts defined 
For purposes of this section, the term adjusted taxable gifts means the total amount of the taxable gifts (within the meaning of section 2503 as modified by section 2511) made by the decedent after December 31, 1976, other than gifts which are includible in the gross estate of the decedent.
(2) Adjustment for certain gift tax 
For purposes of this section, the rules of section 2001 (d) shall apply.