The Straits of Malacca was, and still is, one of the world's major waterways facilitating East-West trade and sea passage. As such, for centuries it has played a unifying and integrative role in insular Southeast Asia, also known as the "Malay World", the Malay Archipelago, and Nusantara. It has offered a medium for socioeconomically and socio-politically interconnecting the Malay Peninsula with the many islands of the Malay Archipelago, particularly Sumatra and Java. During the ascendency of Srivijaya and Melaka (Malacca) in the seventh-thirteenth century CE and the fifteenth CE, respectively, the Straits was paramount not only in unifying the Malay Archipelago under these kingdoms’ political and economic patronage, but also in overseeing and dominating international maritime trade throughout Southeast Asia. This prominent waterway continued in this role until the early decades of the nineteenth century, when the Straits was accorded a dividing function by two Western colonial powers: Britain and the Netherlands. Acknowledged as the "exclusive Lords of the East", these imperialist powers transformed the Straits into a colonial divide, with all territories to the north and northeast of this divide falling under the influence of Britain and lands to its south and southeast being controlled by the Netherlands. To avert tensions that could create a pretext for open armed conflict and resolved the perennially contentious issue of Anglo-Dutch tensions, these powers decided on the partition of the Malay Archipelago through the Treaty of London (1824). This essay shall demonstrate the unifying attributes of the Straits of Malacca from the earliest kingdoms of Southeast Asia until 1824, when the Straits assumed its new function as a Western colonial divide. In this manner, it explains why the previously integrated and unified territorial entities within the Malay World were, as a result of Western imperialism and colonialism, were partitioned into two realms that subsequently produced four nation-states (Malaysia, Indonesia, Singapore, and Brunei), each with its particular historical developments and experiences.