Which separate battle did Grouchy fight while Waterloo was underway?

Grouchy fought the Battle of Wavre while Napoleon’s main army was engaged at Waterloo. The fighting at Wavre took place in Belgium on 18–19 June 1815, as French forces under Marshal Emmanuel de Grouchy confronted the Prussian III Corps commanded by Johann von Thielmann.

Napoleon had ordered Grouchy to pursue the retreating Prussians and prevent them from joining Wellington. Grouchy reached the Wavre area as the cannonade from Waterloo was beginning. His subordinates urged him to march toward the sound of the guns, but he continued attacking the Prussian rearguard at Wavre under his written and verbal instructions.

Wavre ended as a French tactical victory, but it had major strategic consequences. The Prussians held Grouchy’s force in place, preventing roughly 33,000 French troops from reaching Waterloo, while Blücher’s other corps marched to Wellington’s aid. A common mix-up is to treat Wavre as part of the Battle of Waterloo itself; it was a separate, simultaneous engagement in the same campaign.