Complex sentence: A violent agitation for his recall, joined by the Liberal Party leader, Henry Campbell-Bannerman, was organized, however unsuccessfully and, in August, Milner returned to South Africa, plunging into the herculean task of remodelling the administration.
Split into simpler sentences: A violent agitation for his recall was organized,, joined by the Liberal Party leader Henry Campbell-Bannerman. However it was unsuccessful, and in August Milner returned to South Africa, plunging into the herculean task of remodelling the administration.