The United States' Discretional Function Exception Defense

It is a fact that the United States Attorney, legal representative of the United States Government,"may not plead" the defense of Discretional Function Exception "without the prior approval" of a special unit of the government. This is "without exception!" What's the legal significance? This is the United States of America's last line of defense.
Reference: See Government Manual re: Discretional Function Exception Defense

4-5.220 Torts Branch Procedures -- Substantive Considerations in FTCA Litigation
The Assistant United States Attorney assigned to a tort suit is expected to assume full responsibility for preparation of an aggressive, professional defense to the suit, unless the suit is one assigned to be handled directly by a component of the Torts Branch. The initial letter from the Torts Branch will request the agency to forward a litigation report to the Assistant United States Attorney. This litigation report will be the starting point for development of the facts and legal position to be taken in the litigation. However, the Assistant United States Attorney is responsible for ensuring that each reasonable legal and factual defense is pursued regardless of whether the agency litigation report identifies the defense. The Torts Branch Monographs and, particularly the FTCA Staff's Monograph "Checklist of FTCA Defenses," provide assistance.

The Assistant United States Attorney should obtain approval from the appropriate Torts Branch Staff prior to raising the "discretionary function exception" defense in any case and may well desire to consult with the Branch when a difficult issue pertaining to any of the exceptions or exclusions to the Federal Tort Claims Act arises. If the case is designated as a monitored case, Assistant United States Attorney may seek assistance from the Torts Branch attorney or reviewer designated in the initial letter from the Torts Branch to the agency requesting a litigation report. If the case is designated as a delegated case, the author of the appropriate Monograph, if any, should be contacted or inquiry may be made by calling the responsible Director's office.

The FTCA is the exclusive remedy for common law torts committed by federal employees acting in the scope of employment. United States Attorneys are authorized to make the certification required by law (28 U.S.C. § 2679(d)(1)) in order to substitute the United States for a federal employee against whom a common law tort suit is brought. See 28 C.F.R. § 15.3.

4-5.230 Torts Branch Procedures -- Settlement of Federal Tort Claims Act Suits
United States Attorneys responsible for the defense of FTCA or other tort litigation (e.g., Suits in Admiralty Act or Vessels Act) are currently delegated $1,000,000 in settlement authority, subject to the limitations set forth in Civil Division Directive No. 14-95, 28 C.F.R. Part O, Subpart y, App. If a United States Attorney seeks to settle for an amount in excess of the delegated authority, a detailed justification for the settlement must be forwarded to the Torts Branch. The responsible Director will then make a recommendation to the Assistant Attorney General (or if the proposed amount is in excess of $2,000,000 to the Associate Attorney General). Although the Torts Branch endeavors to expedite consideration of settlement proposals, opposing counsel and, if necessary, the court should be informed that immediate action cannot be guaranteed on any settlement proposal.It is customary to consult with the Torts Branch during settlement negotiations when any concern arises regarding the advisability of settlement or of the amount of the settlement.

Although authority to settle a case can be obtained in exceptional cases prior to submission of an authorized offer from the other party(ies) to the case, this procedure is highly disfavored and should not be used unless special justification for its use is provided. However, the Torts Branch will provide counsel as to what amount it will recommend to the Assistant Attorney General in advance of initiation or completion of settlement negotiations. Stipulations for admissions which are tantamount to a stipulation of liability must be approved by whatever level of authority is appropriate based on the highest reasonably predictable judgment that the court could enter predicated upon the stipulation or admission.