The Dress Code is Black Tie.

For military members, active and reserve, the dress code is Dinner Dress Blue Jacket, or their service equivalent.

Black tie is the second highest level of formal dress. It is defined by well-fitting tuxedos and floor-length hemlines, evening fabrics and bow ties.

Dresses should be floor-length or longer (if you're in the mood for a train, go for it!).

Some appropriate fabrics and textures for dresses include: silks and satin, sequins, intricate beading; jacquard and velvet, chiffon, tulle.

Some fabrics that are not black tie: cotton, eyelet lace, limen, chambray, or denim.

For an excellent, simple breakdown of Black Tie for men, here's an article from GQ. Here's another guide to black tie, but written by a Brit.

The simple answer is that a Tuxedo (or a Dinner Suit) has certain details that make it more formal. This article, written by a graphic designer and James Bond filmic style expert, details a more useful explanation.

What Makes a Tuxedo More Formal than a Suit, by Matt Spaisar