how does this all change with the arrival of the Europeans

Basically the social structures, which Okonkwo in particular based his identity on, were systematically dismantled. Christianity decimated their ancient framework of how the Igbo understood their world. Igbo males were, to a large extent, emasculated by the new power of the white man. The European socio-economic value systems changed the importance of farming, yams in particular, and traditional economic stability.