The Free French invaded two small islands near Newfoundland (St. Pierre and Miquelon) that were French (and so Vichy, at the time) possessions. The "why" of this invasion may be a bit more difficult. I would suggest that De Gaulle was looking for an opportunity for the Free French forces to take positive action against the Vichy regime and these islands represented their best immediate chance for success. Moreover, the USA had just entered the war, and although Churchill was strongly behind De Gaulle and the Free French, the USA was not. By landing on islands on the N. American continent, De Gaulle could send a message to both Paris and Washington.