Paul studied at the Ecole Spéciale Militaire de Saint-Cyr and graduated 3rd of 357 in his class.

On 21 October 1914, the division arrived in Flanders, to support the hard-pressed Belgian Army during the Battle of the Yser.

For his notable leadership, Grossetti was given command of the 16th Army Corps on 7 November 1914, in the midst of the First Battle of Ypres.

After the unsuccessful Battle of the Crna Bend (1917), Grossetti was infected by dysentery and was repatriated to France in September 1917.

King Albert I of Belgium had a statue erected in Ajaccio, in gratitude for the intervention by Grossetti's 42nd Division during the Battle of the Yser.