How did Radama I expand Merina control across Madagascar?

Radama I (reigned 1810–1828) expanded Merina control across Madagascar primarily through a strategic alliance with the British governor of Mauritius, Sir Robert Farquhar.12

To prevent French reoccupation of the east coast, Farquhar provided Radama with military advisers and weapons, while formally recognizing him as the "King of Madagascar". Leveraging this support, Radama captured the east-coast town of Tamatave in 1817, using it as a base to launch annual military expeditions against coastal populations.12

Through these campaigns, he successfully conquered most of the east coast, the northern region of the island, and the majority of the two large Sakalava kingdoms. By the end of his reign, he had made tributaries of most of the island's kingdoms, significantly centralizing power under the Merina monarchy.123