As per Free Republic:

However, because of the President's constitutional duty to act for the United States in the field of foreign relations, and his inherent power to protect national security in the context of foreign affairs, we reaffirm what we held in United States v. Clay, supra, that the President may constitutionally authorize warrantless wiretaps for the purpose of gathering foreign intelligence."--United States v. Brown, 484 F.2d 418, 426 (1973)


"We agree with the district court that the Executive Branch need not always obtain a warrant for foreign intelligence surveillance."--U.S. v. Truong Dinh Hung, 629 F.2d 908, 913 (1980)


"Prior to the enactment of FISA, virtually every court that had addressed the issue had concluded that the President had the inherent power to conduct warrantless electronic surveillance to collect foreign intelligence information, and that such surveillances constituted an exception to the warrant requirement of the Fourth Amendment."--United States v. Duggan, 743 F.2d 59 (1984)


"The Truong court, as did all the other courts to have decided the issue, held that the President did have inherent [constitutional] authority to conduct warrantless searches to obtain foreign intelligence information."--In re Sealed Case, 310, F3d. 717, 742 (2002)


"If a war be made by invasion of a foreign nation, the President is not only authorized but bound to resist force by force. He does not initiate the war, but is bound to accept the challenge without waiting for any special legislative authority." --The Amy Warwick (The "Prize Cases"), 67, U.S. 635, 668 (1862)