Which state included Waterloo when the battle was fought in 1815?

Waterloo was fought in the United Kingdom of the Netherlands in 1815.

The battle took place on 18 June 1815 near Waterloo, south of Brussels. At that time, the modern state of Belgium did not yet exist. The area belonged to the United Kingdom of the Netherlands, created in 1815 after Napoleon's defeat and the rearrangement of Europe by the Congress of Vienna.

The coalition army opposing Napoleon included troops from the United Kingdom, the Netherlands, Hanover, Brunswick and Nassau, while Prussian forces arrived during the battle. This mixture can cause confusion: “United Kingdom” in the answer refers to the Dutch-ruled state, not the United Kingdom of Great Britain and Ireland.

Belgium became an independent country only after the Belgian Revolution of 1830. Thus, Belgium is the modern location, but the United Kingdom of the Netherlands is the correct political designation for the battlefield in 1815.