Even if the Portuguese hold on these three African regions was tenuous, they clearly remained the main European presence in Sub-Saharan Africa. It was natural to assert their claim, therefore, in all three regions when the scramble for Africa began later. Prolonged military campaigns were required to retain and impose Portuguese control over the Africans in these territories in the late 19th century. The boundaries of Portuguese Guinea were agreed upon in two stages in 1886 with France, the colonial power in neighbouring Senegal and Guinea. No other nation presented a challenge for the vast and relatively unprofitable area of Angola. The most likely scene of conflict was Portuguese East Africa, where Portugal's hope of linking up with Angola clashed with Britain's plans for the Rhodesias. There was a diplomatic crisis in 1890, but the borders between British and Portuguese colonies were agreed upon by treaty in 1891.