How did Radama I expand Merina control over Madagascar?

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

To prevent French reoccupation of the east coast, the British provided Radama with military advisers and weapons, while formally recognizing him as the "King of Madagascar". In exchange, Radama agreed to cooperate with Britain’s campaign to abolish the slave trade.12

Using these resources, Radama captured the east-coast town of Tamatave in 1817, which served as a base for annual military expeditions against coastal populations. Through these campaigns, he successfully conquered:12
* Almost the entire east coast.12
* The northern region of the island.12
* Most of the two large Sakalava kingdoms.12

By the end of his reign, Radama had made most of the kingdoms on the island tributaries to the Merina state, leaving only parts of the south and west independent.132