We give a simple derivation of the van der Waals free energy of interaction between two semiinfinite media, including effects of nonzero temperature and retardation. The method, an extension of one used by van Kampen and co-workers at the short-distance and low-temperature limits, considers the free energy of electromagnetic surface modes in the region between the two media. The result is the same as derived by Lifshitz and co-workers using Green's function techniques.