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CASSIRER v. THYSSEN-BORNEMISZA COLLECTION 
FOUNDATION 
Opinion of the Court 

just as any other actor would. 

And in so doing, Section 1606 also dictates the selection 
of  a  choice-of-law  rule:  It,  too,  must  mirror  the  rule  that 
would apply in a similar suit between private parties.  For 
only the same choice-of-law rule can guarantee use of the
same substantive law—and thus (see above) guarantee the 
same liability.  See Barkanic v. General Admin. of Civ. Avi-
ation of People’s Republic of China, 923 F. 2d 957, 959–960 
(CA2 1991) (“[T]he same choice of law analysis” is needed 
to  “apply[ ]  identical  substantive  laws,”  and  so  to  “ensure 
identity of liability” between a foreign state and a private 
individual).  Consider two suits seeking recovery of a paint-
ing—one suit against a foreign-state-controlled museum (as 
here), the other against a private museum.  If the choice-of-
law rules in the two suits differed, so might the substantive 
law in fact chosen.  And if the substantive law differed, so 
might  the  suits’  outcomes.  In  one  case,  say,  the  plaintiff 
would  recover  the  art,  and  in  the  other  not.    Contrary  to 
Section 1606, the two museums would not be “liable in the 
same manner and to the same extent.” 

In this case, then, Section 1606 requires the use of Cali-
fornia’s choice-of-law rule—because that is the rule a court 
would  use  in  comparable  private  litigation.    Consider  the 
just-hypothesized suit against a private museum for return
of a piece of art, brought as this case was in California.  The 
claims asserted (again, as in this case) turn only on state or
foreign  property  law,  with  no  substantive  federal  compo-
nent.  If the private suit were filed in state court, Califor-
nia’s choice-of-law rule would of course govern.  And if the 
private suit were filed in federal court, under diversity-of-
citizenship jurisdiction, the same would be true.  According
to long-settled precedent, a federal court sitting in diversity
borrows  the  forum  State’s  choice-of-law  rule.    See  Klaxon 
Co. v. Stentor Elec. Mfg. Co., 313 U. S. 487, 496 (1941).  So 
the private-museum suit would begin with the application 
of California’s choice-of-law rule, to decide on the governing