Soon after the war began the 3rd Division was sent to France where it became one of four divisions forming the initial British Expeditionary Force (BEF), although there was no immediate action. On 29 February 1940, Lathbury, promoted to the acting rank of major, was posted to the 48th (South Midland) Infantry Division as a General Staff Officer Grade 2 (GSO2). The division, commanded by Major General Andrew Thorne, was a first line Territorial Army formation which had only recently arrived in France. The German Army launched their invasion of France just over two months later, and the 48th Division, along with most of the rest of the BEF, was forced to retreat to Dunkirk, from where most of it was evacuated at Dunkirk. For his services in France and Belgium Lathbury was made a Member of the Order of the British Empire. For the rest of the year the division was in South West England on anti-invasion duties.