According to some modern historians, Trajan might have busied himself during his stay on the Persian Gulf with ordering raids on the Parthian coasts,[64] as well as probing into extending Roman suzerainty over the mountaineer tribes holding the passes across the Zagros Mountains into the Iranian Plateau eastward, as well as establishing some sort of direct contact between Rome and the Kushan Empire.[65] No attempt was made to expand into the Iranian Plateau itself, where the Roman army, with its relative weakness in cavalry, would have been at a disadvantage.[66]