James FitzGibbon commanded a group of experienced soldiers and Indians with a mission to harass the enemy. The Americans called this group FitzGibbon's Green'uns as a mark of their respect and fear. The green in the name may have come from the green-grey colour of the fustian jackets they wore over their tunics to make them harder to see among the trees, or it may have come from the nickname of their regiment. FitzGibbon's men were part of the 49th Regiment, called the Green Tigers because of the green facings on their uniform coats.

In late July 1813, Lt.Col. Charles Boerstler of the American Army was ordered to attack FitzGibbon's force, which was then at John DeCew's home at DeCew Falls, near present St Catharines. Some American officers were billeted at the home of Laura Secord, the wife of James Secord, who had been wounded at Queenston Heights. One evening, she overheard the officers talking about the upcoming raid on FitzGibbon. She left home as discreetly as possible and walked 22 miles until she was found by some of FitzGibbon's Mohawks, who were on a scouting mission to locate Boerstler's force. She was taken to FitzGibbon, who probably knew what was happening anyway but was gentlemanly enough to give her credit for the information.

To reach FitzGibbon's headquarters, Boerstler's force advanced from Fort George, then in American hands, to Queenston. From there, the Americans moved along the St David's Road below the escarpment before crossing to the Mountain Road, an old native trail which at that time ran along the edge of the escarpment from Stamford to Beaver Dams, a small community south of Thorold. From Beaver Dams, a road ran directly to DeCew's home. Nowadays Mountain Road stops short, having been blocked by the fourth Welland Canal. The remainder of Mountain Road to the west of the Welland Canal is now called Beaverdams Road. At a point just west of the Thorold Tunnel, Caughnawagas led by Francois Ducharme and Mohawks led by William Kerr had set up an ambush where a gully crossed the Mountain Road. Boerstler fell right into the trap.

Meanwhile, FitzGibbon and his men had remained at DeCew's home in case a second attack came. Hearing the sounds of battle, he hurried along the DeCew Road to Beaver Dams, where DeCew Road joins Beaverdams Road, the former Mountain Road. He then hurried up Beaverdams Road and Pine Street to a small knoll where Chapel and Lynden Streets intersect. From there he could see what was happening. He sent a message to Major DeHaren, advising him of the situation and asking him to move to cut off any retreating Americans.

Meanwhile, the Americans having been hit very hard and Boerstler himself having been wounded twice, were organizing a retreat. They still had a force numerically superior to anything the British and natives could muster, so FitzGibbon decided on a bluff. Attaching a white handkerchief to his sword and with buglers sounding Cease Fire, he rode toward the Americans. By feeding the Americans' fear of the natives, FitzGibbon convinced Boerstler that the British force was far larger than the American and that he might be unable to restrain the natives if the battle continued. Despite a near disaster when Major DeHaren arrived and tried to take charge, Boerstler surrendered and was surprised when he found that he had surrendered to a much weaker force. The whole battle had taken less then three hours and credit was due entirely to the natives. However, as John Norton, a native leader said, "the Caughnawagas did the fighting, FitzGibbon got the glory, and the Mohawks got the plunder."