In 1822, he was charged with aiding the flight of the four sergeants of La Rochelle, but was acquitted. In 1823 he decided to leave France and went to Greece, to help the Greeks during their ongoing War of Independence. His first task was the supervision of the fortifications of Navarino. Then he travelled to Britain to drum up support among the Philhellenes. Returning again to Greece, he was appointed head of the small Greek regular army, with which he participated in several battles, most notably the Siege of the Acropolis of Athens in 1826. In 1828, he returned to France, only to return to Greece alongside the French Morea expedition. For his services during the Greek War of Independence, the Third National Assembly at Troezen declared Fabvier an honorary Greek citizen in 1827, and he was later awarded the Grand Cross of the Order of the Redeemer by King Otho I.