Both the Depression and World War II hit Phi Mu Delta chapters hard. They merged with Delta Alpha Pi in 1934, acquiring three chapters, but these chapters were short-lived due to the Depression. The University of Vermont chapter closed its doors for 21 years beginning in 1936.
The war was especially tough on Phi Mu Delta. While many PMDs enlisted and fought in the war, their chapters crumbled away. As many as 800 Phi Mu Deltas took part in the war effort, and about 90 made the supreme sacrifice. The last chapter casualty of this period was the Connecticut Chapter, which remains closed.