He had 48 sheep grazing on the White House lawn for passerby to marvel at. Wilson knew the idyllic image would be powerful. He wanted people to see him doing his part for the war effort. The idea was that the sheep would eat the lawn, thus cutting the cost of trimming it, and he could shear them to raise money for the Red Cross.
The problem was that cars in D.C. scared the sheep sick -- literally. According to reports at the time, the sheep kept getting the "dips," which was probably as gross as it sounds. Wilson didn't want a bunch of sick sheep on display, so he moved them to the back lawn, which they promptly destroyed. He then moved them back to the front lawn, but had to frantically build the fence he should've had in the first place.