Athens became the leading city of ancient Greece in the first millennium BC, with its cultural achievements during the 5th century BC laying the foundations of Western civilization. During the Middle Ages, the city experienced decline and then recovery under the Byzantine Empire, and was relatively prosperous during the Crusades, benefiting from Italian trade; after a long period of decline under the rule of the Ottoman Empire, Athens re-emerged in the 19th century as the capital of the independent Greek state, and in 1896 hosted the first modern Olympic Games. In the 1920s a number of refugees, expelled from Asia Minor after the Greco-Turkish War (1919-1922), swelled Athens' population; nevertheless it was most particularly following the Second World War, and from the 1950s and 1960s, that the population of the city exploded, and Athens experienced a gradual expansion in all directions. In the 1980s it became evident that smog from factories and an ever increasing fleet of automobiles, as well as a lack of adequate free space due to overcongestion, had evolved into the city's most important challenges. A series of anti-pollution measures taken by the city's authorities in the 1990s, combined with a substantial improvement of the city's infrastructure (including the Attiki Odos ring road, the dramatic expansion of the Athens Metro, and the brand new Athens International Airport), alleviated pollution considerably and transformed Athens into a much more functional city.