compensation for her injuries. Wake sought monetary damages of ten million dollars on each count, as well as certain injunctive relief, costs and attorney’s fees.
On March 13, 1995, the district court dismissed all claims against the federal defendants on the authority of Feres. Wake’s claims in Counts II, III, and IV, insofar as they implicate Norwich University and unknown members of the Norwich University faculty, remain pending in the district court. Norwich University moved to amend its answer principally to assert a cross-claim against the federal defendants on March 21, 1995, seeking indemnification for any vicarious liability imposed on the University as a result of Etiinger’s negligence. Thereafter, the federal defendants moved for entry of partial judgment pursuant to Fed.R.Civ.P. 54(b). On July 21, 1995, the district court denied the University’s motion to amend its answer, but granted the federal defendants’ motion for entry of partial judgment.
This appeal followed.
DISCUSSION
On appeal, Wake contends principally that the district court erred in granting the federal defendants’ motion under Fed.R.Civ.P. 12(b)(6) and dismissing all claims for relief against the federal defendants under the FTCA for lack of subject matter jurisdiction under the Feres doctrine. In addition, Norwich University contends principally that the district court erred in denying its motion to amend its answer to assert an indemnification claim against the federal defendants.
As an initial matter, we believe that the proper vehicle for dismissing a Feresbarred FTCA claim is a dismissal for lack of subject-matter jurisdiction. See Fed. R.Civ.P. 12(b)(1). The FTCA was intended as a partial waiver of sovereign immunity. Feres, 340 U.S. at 140, 71 S.Ct. at 156. “ ‘Absent a waiver, sovereign immunity shields the federal Government and its agencies from suit_ [Thus, sovereign immunity is jurisdictional in nature.’” Dorking Genetics v. United States, 76 F.3d 1261, 1264 (2d Cir.1996) (quoting FDIC v. Meyer, 510 U.S. 471, -, 114 S.Ct. 996, 1000, 127 L.Ed.2d 308 (1994)). Because the Feres doctrine concerns the waiver of sovereign immunity, see Feres, 340 U.S. at 146, 71 S.Ct. at 159, a question of whéther a FTCA claim is barred by Feres is necessarily one of jurisdiction. See Sanchez v. United States, 878 F.2d 633, 634 (2d Cir.1989) (“Sanchez IIP) {Feres dismissal for lack of jurisdiction). Accordingly, because of. the importance of the jurisdictional issue, see Fed R. Civ. P. 12(h)(3), we will treat the Government’s Rule 12(b)(6) motion as a jurisdictional challenge under Rule 12(b)(1), see 5A Charles Alan Wright & Arthur R. Miller, Federal Practice and Procedure § 1350, p. 205 (2d ed.1990).
“When reviewing a district court’s [Rule 12(b)(1) ] determination of its subject matter jurisdiction, we review factual findings for clear error and legal conclusions de novo.” In re Vogel Van & Storage, Inc., 59 F.3d 9, 11 (2d Cir.1995). We review the denial of a motion to amend for abuse of discretion. See Prins v. Coughlin, 76 F.3d 504, 507 (2d Cir.1996) (per curiam).
(1) The Feres Doctrine
In Feres, -the Supreme Court held that the United States is not hable for injuries to members of the military under the FTCA if their “injuries arise out of or are in the course of activity incident to service.” 340 U.S. at 146, 71 S.Ct. at 159. Three rationales underlie the Feres doctrine: (1) the “distinctly federal” relationship between the Government and members of its armed forces; (2) the existence of a uniform system of “generous statutory disability and death benefits” for - members of the military; and (3) the need to preserve the military disciplinary structure and prevent judicial involvement in sensitive military matters. See United States v. Johnson, 481 U.S. 681, 688-691, 107 S.Ct. 2063, 2067-69, 95 L.Ed.2d 648 (1987); see also United States v. Stanley, 483 U.S. 669, 682-683, 107 S.Ct. 3054, 3063-64, 97 L.Ed.2d 550 (1987). As we have recently observed, the three broad considerations underlying -the Feres doctrine permeate the process of assessing whether the totality of the germane facts giving rise to a service member’s claim occurred within the purview of a distinctly military sphere of activity or