The consequence of the studies initiated by the US and subsequent consultations between the Major Allies was that the US government extended an invitation to 55 States or authorities to attend, in November 1944, an International Civil Aviation Conference in Chicago. Fifty-four States attended this Conference end of which a Convention on International Civil Aviation was signed by 52 States set up the permanent International Civil Aviation Organization (ICAO) as a means to secure international co-operation an highest possible degree of uniformity in regulations and standards, procedures and organization regarding civil aviation matters. At the same time the International Services Transit Agreement and the International Air Transport Agreement were signed.
 
The most important work accomplished by the Chicago Conference was in the technical field because the Conference laid the foundation for a set of rules and regulations regarding air navigation as a whole which brought safety in flying a great step forward and paved the way for the application of a common air navigation system throughout the world.
 

PICAO - North Atlantic Route Service Conference (Dublin, March 1946)
Because of the inevitable delays in the ratification of the Convention, the Conference had signed an Interim Agreement, which foresaw the creation of a Provisional International Organization of a technical and advisory nature with the purpose of collaboration in the field of international civil aviation (PICAO). This Organization was in operation from August 1945 to April 1947 when the permanent ICAO came into being. Its seat was in Montreal, Canada and in 1947 the change from PICAO to ICAO was little more than a formality. However, it also brought about the end of ICAN because, now that ICAO was firmly established, the ICAN member States agreed to dissolve ICAN by naming ICAO specifically as its successor Organization.
 
From the very assumption of activities of PICAO/ICAO, it was realized that the work of the Secretariat, especially in the technical field, would have to cover two major activities:
  • those which covered generally applicable rules and regulations concerning training and licensing of aeronautical personnel both in the air and on the ground, communication systems and procedures, rules for the air and air traffic control systems and practices, airworthiness requirements for aircraft engaged in international air navigation as well as their registration and identification, aeronautical meteorology and maps and charts. For obvious reasons, these aspects required uniformity on a world-wide scale if truly international air navigation was to become a possibility. Activities in these fields had therefore to be handled by a central agency, i.e. ICAO headquarters, if local deviations or separate developments were to be avoided;
  • those concerning the practical application of air navigation services and facilities by States and their coordinated implementation in specific areas where operating conditions and other relevant parameters were comparable.
To meet the latter objective it was agreed to sub-divide the surface of the earth into a number of "regions" within which distinct and specific air navigation problems of a similar nature existed. A typical example of this process is illustrated by a comparison of the so-called "North Atlantic Region (NAT)", where the primary problems concern long-range overseas navigation, with the "European-Mediterranean region (EUR)" where the co-ordination of trans-European operations with domestic and short-range international traffic constitutes the major problem. Once the regions created, it was necessary to provide bodies which were able to assist States in the resolution of their specific "regional" problems and it was agreed that this could best be achieved by the creation of a number of Regional Offices which were to be located either in the Region they served or, if more than one Region was to be served by such an Office, as close as possible to the Region concerned.
 
As a consequence of the above ICAO adopted the concept of Regions and Regional Offices on the understanding that any regional activities could only be undertaken provided they did not conflict with the world-wide activities of the Organization. However, it was also recognized that such activities could vary from Region to Region taking into account the general economic, technical or social environment of the Region concerned.

Mr. E. Warner, First President of the Council of ICAO From 1947 to 1957