Which coalition opposed Napoleon at Waterloo?

The coalition that opposed Napoleon at Waterloo was the Seventh Coalition.

Formed in 1815 after Napoleon escaped from Elba and returned to power, it united European states determined to remove him again. At Waterloo, the decisive forces were Wellington’s Anglo-allied army and Blücher’s Prussian army, supported by contingents from the Netherlands, Hanover, Brunswick and Nassau. Other coalition powers, including Austria and Russia, were mobilising but did not fight at Waterloo itself.

The battle on 18 June 1815 was part of the War of the Seventh Coalition, also called the Hundred Days campaign. Napoleon hoped to defeat Wellington and Blücher separately before the larger coalition armies could invade France. Instead, the Prussians reached the battlefield during the afternoon and helped turn the engagement against the French.

A common mix-up is the Sixth Coalition, which defeated Napoleon in 1813–14 and forced his first abdication. Waterloo belonged to the next, Seventh Coalition, formed after his return from exile.