Marengo, Battle of, June 14, 1800, a French victory in the Napoleonic Wars. It was fought in the Piedmont region of northwest Italy. Baron Melas, the Austrian commander, launched a surprise attack with 30,000 troops against 23,000 French led by Napoleon Bonaparte. The French were near defeat until the arrival of 6,000 reinforcements. Melas agreed the next day to remove Austrian troops from most of northern Italy.