Complex sentence: By 1940 the strain on the marriage had become too great and at a double celbration for Rena's birthday and their anniversary (June 7), Borzage (who had developed a drinking problem) suddenly left his mansion and moved out.
Split into simpler sentences: By 1940 the strain on the marriage had become too great. At a double celebration for Rena's birthday and their anniversary on June 7, Borzage, who had developed a drinking problem, suddenly left his mansion and moved out.