IN APRIL, WHILE THE UNITED STATES MAINTAINED A . . .

In April, while the United States maintained a defensive posture in the Pacific, Japan, according to a three-fold war plan, prepared to move toward new conquests: to Tulagi in the lower Solomons, and Port Moresby, southernmost Allied outpost in New Guinea, or the purpose of achieving air supremacy in the Coral Sea; to Midway and the Western Aleutians for the purpose of strengthening her defense perimeter and forcing a decisive fleet engagement with the United States Navy; and the the Nw Caledonia-Samoa chain for the purpose of severing the line of communications between the United States and the Anzac nations.  The enemy was soon challenged in the first two areas, and as a result the third operation never came about.
     The Imperial Japanese Navy began Operation "Mo," the first offensive, with carriers  Shokaku and Zuikaku, borrowed from Nagumo's Carrier Striking Force based in Ceylon and from Vice Admiral Inouye's Fourth Fleet based at Truk and Rabaul.  The rest of the "Mo" force consisted of a Tulagi Invasion Group, a Support Group bound for the Louisades, and a joint Covering Group.  Overall command was exercised by Inouye in Rabaul, near the northern extremity of the Solomons.
     Fortunately United States Army cryptographers, working closely with Naval Intelligence, had broken Japan's secret code and as a result Nimitz by April 17 knew the enemy's precise intentions.  After hastily conferring with MacArthur, who was able to supply about three hundred Allied land-based aircraft, it was decided that this was indeed a major enemy thrust and it was to be met with all available military power.
     The Battle of the Coral Sea which followed, the first exclusively carrier-air conflict of the war, is told if four parts: the preliminaries by Nimitz and naval historian E. B. Potter, Chairman of the Naval History Department at the U. S. Naval Academy, and the other parts by men who participated in the battle.

--S. E. Smith
From: The United States Navy in World War II
Preface to: Part III: Chapter 3: Coral Sea Preliminaries