In 1893 he worked for the candidacy of René Viviani as Deputy in the 5th arrondissement of Paris and began to write for the socialist journal La Petite République.

With the outbreak of World War I (1914–18) he was made a temporary Military Engineer (2nd class) and spent the next four years studying the manufacture and use of explosives, munitions and rockets.

[2] On 3 September 1918 he filed a patent application for a "shell for firing against armor plating and resisting targets".

[5] In a meeting of 27 October 1920 the Superior Council of National Defense decided unanimously to reduce the term of military service to 18 months.

Lefèvre presented the project to the Council of Ministers who approved it on condition that parliament accepted two years of service for a transitional period.

On 14 December 1920, when Lefèvre was taking the waters at Vichy, the government submitted the bill on recruitment and reorganization of the army, reducing the number of divisions that could be immediately mobilized.