Embassy shindigs are invariably painful events. The booze is however plentiful and you will find most of the folks who are not on the diplomatic "cocktail party circuit" hanging out near the bar or serving station. Decent place for making some new contacts.

Dark suit, dress uniform or national dress are a must. I'm surprised the invite didn't mention it. I'm going through some old invitations (I keep the nicer embossed ones in a box to impress my mother) and every single one has a Dress Code listed at the bottom right below the RSVP details. The British High Commission and Dutch Embassy ones invariably have the most formal dress code, with the Americans going as low as "business casual" (oh, the horrors) for a "brunch" event.

One tip for these events is to beware of the security restrictions (if in doubt, ask at the time of RSVP) in effect. Car parking may be prohibited for a number of blocks nearby so you may want to plan alternative arrangements with a car service or taxi company. Mobile phones and other small electronics may also be restricted depending on the threat level so plan accordingly.