the FSIA. 28 U.S.C. § 1603(a), (b); see World Wide Br. at xiii. As such, like Kazakhstan, the State Committee is entitled to immunity unless one of the statute’s enumerated exceptions applies. Id: § 1604. 9 . This was so despite the .fact that Kazatom-prom devoted a substantial portion of its brief to arguing against the applicability of the commercial activity exception. See Kazatom-prom Br. at 14-24. 10 . The defense of sovereign immunity may be raised at any time because, if valid, it means that the court lacks power to hear the case. See Steel Co., 523 U.S. at 94-95, 118 S.Ct. at 1012-13. A challenge to. sovereign immunity, by contrast, is an argument that can be waived. See Watters v. Washington Metro. Area Transit Auth., No. 01-7092, slip. op. at 7 n.13 (D.C.Cir. July 17, 2002) (<HOLDING>). 11 . As we have previously noted, the “FSIA
holding that claims made but not argued in a brief are waived holding that a claim that sovereign immunity has been waived is itself waived if not argued on appeal holding assignments of error not argued on brief are waived for purposes of appeal holding that an issue not raised on appeal is waived recognizing that immunity may be waived