Meanwhile, the British were mobilizing both local Canadian militia and British garrison troops to defend against the impending invasion of Canada. The Fenians night-marched north across Black Creek (Ontario) through a cedar swamp, then turned inland on Ridge Road on the morning of June 2; taking up a defensive position on Limestone Ridge near the present Canadian town of Ridgeway. There, they clashed with 850 advancing Canadian militia commanded by Lieutenant Colonel Alfred Booker of the 13th Battalion.

In the first hour of the battle, the Canadians appeared to prevail, driving Fenian skirmishers back across Bertie Road. Then something went wrong: to this day, it is not clear exactly what. Some sources say that the Canadian militiamen mistook Fenian scouts on horseback for cavalry. Orders to form a square to defend against a cavalry charge, although quickly countermanded, led to chaos in the Canadian ranks and Booker ordered a withdrawal after ninety minutes of battle. Other sources indicate that troops mistook a company of redcoated 13th Battalion infantry for British troops relieving them and began to withdraw; which then triggered a panic among other troops who mistook the withdrawal for a retreat. O'Neill, observing the chaos breaking out in the Canadian ranks, quickly ordered a bayonet charge that completely routed the inexperienced Canadians.

The Fenians took and briefly held the town of Ridgeway. Then, expecting to be overwhelmed by British reinforcements, they quickly turned back to Fort Erie where they fought a second battle, the Battle of Fort Erie, against a small but determined detachment of Canadians holding the town.