The royal family has a long association with the military. Diana herself held several honorary military appointments in Australia, Canada and the United Kingdom, including being Colonel-in-Chief of the 13th/18th Royal Hussars (Queen Mary's Own). She was forced to relinquish these positions upon her divorce. Occasionally she would incorporate military elements into her outfits, with somewhat mixed results. However, that is not the case here.
It is described in the Christie's auction catalogue as A Red and Black Dinner Dress Inspired by the Military by Catherine Walker
The bodice is sleeveless with a high neck in Hussar-style, trimmed with black braid and rouleaux. The long skirt, of pleated scarlet silk crepe, is trimmed with scarlet silk soutache brand and rouleaux where it meets the bodice.
In May 1995 it was worn at the Royal Albert Hall for Red Cross concert to commemorate VE Day. The red and black colours of the British Red Cross, which Diana was Vice-President until she resigned in July 1996. She continued to support the organization, notably during her humanitarian trip to Angola in January 1997.
It was Lot 12 at the Christie's dress auction in 1997, where it sold for $36,800 to the Meijiro Fashion and Art College in Tokyo. They also own the Victor Edelstein Victorian inspired ballgown and Yuki Torimaru dresses. The College displays them on special occasions and, from time to time, lend them to Kensington Palace for fashion exhibits.
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Sources
Christie's auction catalogue: DRESSES from the Collection of Diana, Princess of Wales