Complex sentence: During these years, he showed his expertise in economics as well as in leadership by forging relations between anti-fascist groups in order to build a web that formed the Partito d'Azione, over which he presided as a founder.
Split into simpler sentences: During these years, he showed his expertise not only in economics but in leadership as well. There he forged relations between anti-fascist groups in order to build a web that formed the Partito d'Azione, over which he presided as a founder.