April: The Valley of Geysers is discovered on the Kamchatka Peninsula of Russia by Tatyana Ustinova.

April 4: Axis forces capture Benghazi.

April 6: Germany invades Yugoslavia and Greece.

April 9: The U.S. acquires full military defense rights in Greenland.

April 10: The U.S. destroyer USS Niblack, while picking up survivors from a sunken Dutch freighter, drops depth charges on a German U-boat (the first "shot in anger" fired by America against Germany).

April 12: German troops enter Belgrade.

April 13: The Soviet Union and Japan sign a neutrality pact.

April 15: Axis forces reach Halfaya Pass on the Libyan-Egyptian frontier.

April 17: The Yugoslav Royal Army capitulates.

April 18: Prime Minister of Greece Alexandros Koryzis commits suicide as German troops approach Athens.

April 19: Bertolt Brecht's anti-war play Mother Courage and Her Children (German: Mutter Courage und ihre Kinder) receives its first theatrical production at the Schauspielhaus Zürich.

April 21: Greece capitulates. Commonwealth troops and some elements of the Greek Army withdraw to Crete.

April 23: The America First Committee holds its first mass rally in New York City, with Charles Lindbergh as keynote speaker.

April 25: Franklin D. Roosevelt, at his regular press conference, criticizes Charles Lindbergh by comparing him to the Copperheads of the Civil War period. In response, Lindbergh resigns his commission in the U.S. Army Air Corps Reserve on April 28.

April 27: German troops enter Athens. (source: Wikipedia)