When the board of trustees of the Corcoran Gallery of Art held its quarterly meeting on April 20, 1945, the members knew they had a problem. Item No. 15 on the agenda was titled rather blandly, given its implications: “Report, National Gallery — department for contemporary American Art.” C. Powell Minnigerode, the Corcoran’s longtime director, and Jeremiah O’Connor, the veteran curator, would explain.