Lt Col William Tomkinson 16 Light Dragoons, quoted in Kristine Hughes’ Waterloo Winesses page 113, answering criticism of Wellington’s attendance at the Duchess of Richmond’s Ball:
”Something has been said of the headquarters at Brussels thinking so little of the enemy’s advance that they were at a ball, and considerable delay occasioned in consequence. If an army receives information at night which requires an immediate move of troops, it is of great consequence to have the Quartermaster General and other staff officers collected, in place of having to seek and assemble them over a large town. There was considerable time saved by this, and no objection to attending a ball twenty miles from an enemy.”
So, essentially it wasn’t a party, it was a work event….