The major event in 1939 that reshaped alliances in Europe dramatically was the outbreak of World War II following Germany's invasion of Poland. This aggression prompted Britain and France to declare war on Germany, marking the start of the conflict in Europe. The invasion exposed the failure of appeasement policies and fully mobilized military alliances, including the formation of the Axis and Allied powers. As nations began to choose sides, this event also drastically altered geopolitical dynamics, leading to the formation of new alliances and escalating tensions globally. The war ultimately resulted in unforeseen changes in territorial borders and international relations.