Complex sentence: Porto first participated in international club competitions in 1956, when they competed in the second edition of the European Cup, after winning their fourth Primeira Liga title in the previous season.
Split into simpler sentences: Porto first participated in international club competitions in 1956, when they took part in the second edition of the European Cup. Qualification for this competition--contested between the champions of each country--was secured after Porto won their fourth Primeira Liga title in the previous season.