They weren't absent. By Spring of 1940 they had all been invaded by Nazi Germany. But by that time, fighting forces and air defences of Canada, Australia, South Africa, free Poles and free French were already established in Britain to reinforce the country and to begin taking the fight to Germany. It would take another long 1.5 years for the US to join the fight. And even then it took 3.5 more years, and the combined efforts of all, to defeat the advanced and determined forces of Hitler's Nazi regime and its axis allies in Europe. Many aren't aware of this but the first major offensive and land invasion of Nazi Occupied Europe happened nearly a year before D-Day in southern Italy. Led by American and British commanders, US, Canadian and (Polish?) troops landed in southern Italf and began the fight toward Rome.