Complex sentence: Following the Armistice of 11 November 1918, he was put at the disposition of the allied chief commander of the Orient, he joined then the headquarter staff as chief of the first bureau in the beginning of 1919.
Split into simpler sentences: Following the Armistice of 11 November 1918, he was put at the disposition of the Allied Chief Commander of the Orient. He then joined the general staff headquarter as chief of the first bureau in the beginning of 1919.