04/06 Sun. German troops invade Yugoslavia and Greece. Italy declares war on Yugoslavia. 04/07 Mon. United States Naval Operating Base, Bermuda, is established. 04/09 Wed. Battleship NORTH CAROLINA (BB-55) is commissioned at New York, N.Y. Secretary of State and Danish Minister to the United States sign "Agreement Relating to the Defense of Greenland." 04/10 Thu. Destroyer NIBLACK (DD-424) while rescuing survivors of torpedoed Netherlands freighter, depth charges German submarine off Iceland; this is believed to be the first action of the war between United States and German vessels. President proclaims that the Red Sea and Gulf of Aden are no longer combat areas and are open to United States shipping. 04/13 Sun. Russo-Japanese Non-Aggression Pact signed. 04/17 Thu. Egyptian steamship ZAMZAM is sunk by German raider, ATLANTIS, in South Atlantic Area; about 150 Americans are included among rescued passengers. Yugoslavia capitulates to Axis. 04/22 Tue. Authorized enlisted strength of regular Navy is increased to 232,000. 04/23 Wed. Greece signs armistice with Germany 04/24 Thu. Neutrality Patrol is extended eastward to 26 d. West Longitude. 04/26 Sat. Neutrality Patrol is ordered to be extended southward to 20 d. South Latitude. 04/27 Sun. American-Dutch-British Conference at Singapore ends, having reached agreement on combined operating plan of local defense forces in the event of war with Japan; Capt. W. R. Purnell, USN, is senior United States representative.