Because it was a long and slow process, the concrete causes of the decline of the Aksumite kingdoms are inconspicuous. The underlying cause of its decline is the shift of power southward. After the Persians ended Ethiopian involvement in southern Arabia and the Islams replaced the Aksumites in the Red Sea, Amda Tseyon's and Zara Yakob's campaigns into southern lands proved to be permenent settlements.
Until Adulis suffered a destructive attack in the middle of the 7th century, the Aksumites exported ivory, incense, tortoise-shell, and obsidian and they imported clothing, glass, iron, and weapons from Egypt, India, and Arabia (Pankhurst 23). But as Islamic expansions grew in the Red Sea and overpopulation and over-cropping wore down the once fertile land, Aksum's presence in the seashores began to diminish. Trade with other countries, however, did continue, but trading ports that were located much more southern had become more prominent.
The power shift into southern regions had become necessary once deforestation and degradation had taken its toll on the land. While forests were being cut down for construction and irregular rainfall eroded the soil, Aksumite agriculture began to collapse. The power shift southwards was also influenced by revolts occurring in surrounding areas, most notably by the Beja tribes from the north. Ethiopia's expeditions into south Arabia territories played a role in weakening its troops (Henze 44-6).