Why did the Sakalava empire weaken after 1810?

The Sakalava empire, which once controlled much of western Madagascar, experienced a decline due to several compounding factors.

Beyond internal power struggles for the throne and attempts to replace the traditional ancestral cult with Islam, the empire was significantly weakened after 1810 by military conflicts with the Merina people. The Merina, based on the central plateau, had been unified and strengthened under the rule of Andrianampoinimerina (reigned 1787–1810). By selling slaves to the French in exchange for guns, the Merina kingdom developed the military capacity to expand its influence and challenge the Sakalava state, which had already begun to decline during the late 18th century.123