Lassalle obtained a Doctorate of Law, then joined the Ministry of Finance and had risen to office manager at the start of World War I (1914–18).

Although excused from military service due to the loss of an eye, he enlisted in August 1914 as a private in the 49th infantry regiment and volunteered for the front.

[2] Lassalle was made Honorary President of the National Federation of Republican fighters, and founded the Landes branch of this organization.

[3] At the start of World War II Lassalle was offered the grade of colonel, but despite his age enlisted as a lieutenant gunner in the 213th Infantry Regiment.

During leave periods he returned to the Chamber, where he spoke for the last time on 15 March 1940 on the system of military pay and allowances for reserve NCOs.