Complex sentence: In the first years of the party's existence, the PNR was, above all, an instrument Calles,' Maximum Chief' of the party, used to continue exercising power in an era known as the Maximato.
Split into simpler sentences: In the first years of the party's existence, the PNR was, above all, the political machine of Calles. As' Maximum Chief' of the party, he continued to hold the real power in an era known as the Maximato.