II, then Secretary of State Cordell Hull issued regulations declaring that outstanding passports, together with passports issued thereafter, could not be used for travel to Europe without specific validation by the Department of State, and such validation could not last more than six months. Similar restrictions can still be invoked upon notice given in the Federal Register, and such notice was issued in 2017, so that passports were "declared invalid for travel to, in, or through the DPRK unless specially validated for such travel." As confirmed in Haig v. Agee (1981), the administration may deny or revoke passports for foreign policy or national security reasons at any time, and for other reasons as prescribed by regulations. A notable example of enforcement of this was the 1948 denial of a passport to U.S. Representative Leo Isacson, who sought to go to Paris to attend a conference as an observer for the American Council for a Democratic Greece, a Communist front organization, because of the group's role in opposing the Greek government in the Greek Civil War. Denial or revocation of a passport does not prevent the use of outstanding valid passports. The physical revocation of a passport is often difficult, and an apparently valid passport can be used for travel until officially taken by an arresting officer or by a court. The lack of a valid passport (for whatever reason, including revocation) does not render the U.S. citizen either unable to leave the United States, or inadmissible into the United States. The United States is a signatory of the International Covenant on Civil and Political Rights, which guarantees residents of its signatories wide-ranging rights to enter or depart their own countries. In Nguyen v. INS, the Supreme Court stated that U.S. citizens are entitled "...to the absolute right to enter its borders." Lower federal courts went as far as to declare that "...the Government cannot say to its citizen, standing beyond its border, that his reentry into the land of his allegiance is a criminal offense; and this we conclude is a sound principle whether or not the citizen has a passport, and however wrongful may have been his conduct in effecting his departure." Therefore, even in the absence of a valid passport, U.S. citizens are not denied entry into the United States, though these travelers may be delayed while the CBP attempts to verify their identity and citizenship status. The