With the prolonged drought, the rise of Gaza, the dominance of the slave trade, and the expansion of Portuguese control in the Zambezi Valley, the once-mighty African chieftaincies of the Zambezi region declined. In their place, valley warlords established fortified strongholds at the confluence of the major rivers, where they raised private armies and raided for slaves in the interior. The most powerful of these warlords was Manuel António de Sousa, also known as Gouveia, a settler from Portuguese India, who by the middle of the 19th century controlled most of the southern Zambezi Valley and a huge swath of land to its south. North of the Zambezi, Islamic slave traders rose to power from their base in Angoche, and the Yao chiefs of the north migrated south to the highlands along the Shire River, where they established their military power.