after this he became a liaison officer with the british, serving on 1st division of cuirassiers which was part of a multi-national combined military operation that also included the british expeditionary force, and which had been assembled to defend france's northern frontier against a possible german attack through belgium (as had happened in 1914 at the start of the first world war). the anticipated german invasion of belgium and luxembourg was eventually launched on 10 may 1940. hettier de boislambert took part with a tank unit in the fighting around tienen and in which french and british forces attempted to resist the german advance towards the english channel. then he was ordered to rejoin the main french army which was consolidating a defensive frontline along the somme. because of the way the battle lines had moved this now involved crossing enemy lines as the german invasion forces continued their push towards the sea. hettier de boislambert rejoined the french army at the somme frontline on 20 may 1940.