The German army launched an invasion of Poland on 1 September 1939. The and British governments reacted by declaring war on Germany on 3 September 1939, albeit without becoming militarily involved in Poland, even after the Soviet army launched its own invasion of Poland from the other side on 17 September 1939. The French army was more active closer to home, however, and Claude Hettier de Boislambert was called up in September 1939 as part of a generalised mobilisation: he was to serve as a cavalry lieutenant. Immediately he was sent to Lorraine to take part as leader of a horse-back platoon in a reconnaissance group. During the brief invasion of Germany which took place from eastern France in September/October 1939, his reconnaissance group was one of the first to undertake some of the many patrols that the French army high command ordered in the occupied Saarland, notably in the Sierck sector adjacent to the Luxembourg frontier.