The authority of alternates to become or to continue to serve as acting officers, as provided in sections 84-1101 to 84-1116, shall not exist unless and until an attack upon the United States actually has occurred; and, except as otherwise provided in this section, shall terminate upon the expiration of two years after the inception of the attack. The Governor, by proclamation, or the Legislature, by resolution approved by the Governor, may terminate the authority of alternates at some earlier date, and from time to time may extend or restore such authority, upon a finding that events and circumstances at that time render the extension or restoration necessary; Provided, that no single extension or restoration shall be for a period of more than one year.