Complex sentence: The G20 was foreshadowed at the Cologne Summit of the G7 in June 1999, and formally established at the G7 Finance Ministers' meeting on 26 September 1999 with an inaugural meeting on 15--16 December 1999 in Berlin.
Split into simpler sentences: The G20 was foreshadowed at the Cologne Summit of the G7 in June 1999, and was only formally established at the G7 Finance Ministers' meeting on 26 September 1999. The inaugural meeting took place on 15--16 December 1999 in Berlin.