In 1902, Barrère negotiated a secret accord with Italian Minister of Foreign Affairs Giulio Prinetti that ended both countries' historical rivalry for North Africa.

The Prinetti-Barrère Accord stated that in the event of a redistribution of Ottoman lands in Northern Africa, France would not contest an Italian claim on the lands of the Tripolitania Vilayet, which makes up modern Libya.

In return, Italy would not contest a French claim on the Ottoman territory of Morocco.

The agreement allowed for the French Agadir Crisis in Morocco in 1911 and the Italo-Turkish War of 1911–1912, which resulted in the taking of both territories.

Barrère also was a key figure in arranging the 1915 secret Treaty of London between Italy and the Triple Entente, which resulted in Italy abandoning its Triple Alliance partners of Germany and Austro-Hungary during the First World War.