During the first world war he worked for the defence of France, and at this time he fell out with Borel who was doing a similar task. Lebesgue held his post at the Sorbonne until 1918 when he was promoted to Professor of the Application of Geometry to Analysis.
Sa unang mundo digmaan ay nagtrabaho siya para sa depensa ng France, at sa oras na ito siya itumba out sa Borel na noon ay paggawa ng isang katulad na gawain. Lebesgue gaganapin sa kanyang post sa Sorbonne hanggang 1918 kung kailan siya ay itaas na Professor ng Application ng geometry sa Pagtatasa.