It was originally Somali territory. There were already Somalis living on the southern side of the Juba, and although they were smaller in number, they were strengthened by the newcomers who crossed the river and joined them through sheegad alliances.
Being “clients” doesn’t mean they were subservient, it means there were commercial and cooperative relations, especially in grazing and trade. That was the state of affairs prior to the conquest.
A grand scheme? Unrelated? Lmaoo the Bardheere Sultans literally called for Jihaad, and this did not begin in 1860. It began as early as 1836 and continued through the 1890s.
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How can you say it began in 1860 when Somalis had already crossed into the region in the 1830s?
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Also, various southern Raxaweyn tribes, especially those connected to the walled city of Luuq and Bardere, defeated the Wardai/Oromo/Boran, pushing them further south and westwards.
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At. At this point, the Ogaden were already assisting them, further evidence that this was a coalition of Somali clans, not just one group acting in isolation.
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To reinforce the multi-clanal dimension, it wasn’t just Ogaden/Darood working with Raxanweyn. During the conquest, they also sought assistance from coastal clans, including those from the Benadir region.
When the war reached as far as the Tana River, this continued:
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You mention “small gradual migrations” but this was an organized military campaign, not a passive trickle. These were war operations that involved transporting large numbers of Somalis overland, many of whom were mobilized specifically for this campaign.
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By 1889, Oromos were fleeing ahead of the incoming Somali forces, seeking refuge with the Witu Sultans:
''In turn, there was no milk to be had in Witu on the day of the Somalis' arrival
because the Oromo, "notwithstanding that they were under the protection of the Sultan, had fled in panic terror into the woods, with their herds, before the Somalis." These Somalis were part of a group which had carried on war expeditions even across the Tana. Peters wisely made a treaty of peace with Sharif Hussein.''
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They would attack various positions even the European stations where they had Oromos and others under their protection.
''Somalis attack on an East African Mission station"
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So again, this was a structured conquest, not an opportunistic skirmish.
There is an entire recent study on it that southern Somali speakers lived all the way down to Tana River.
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It actually futher strenghtens E.R Turtons own research and reconstruction of local histories
E. R. Turton, Bantu, Galla and Somali Migrations in the Horn of Africa: A Reassessment of the Juba/Tana Area, The Journal of African History, Vol. 16, No. 4 (1975), pp. 519-537
www.jstor.org
The first and most startling feature of this outline is that the Somali preceded the Galla in the Juba/Tana region, for pre-Hawiya Garre moved to Afmadu and the area to the south prior to the migration of the Orma to
the coast. This of course places the nineteenth-century Somali advance
southward across the river Juba into an altogether different perspective,
since it can now be seen as partly a reconquest of lands they had previously held
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