How so? The first isn't a classic slave raid sort of deal; it was also a transactional mercantile relationship (with the Ottomans and the other powers of the Middle East, which the Crimean Khanate founded its entire economy upon) funded by European-on-European warfare. True, there was a lot of difference between the Tatar Europeans and the Slavic Europeans- but there were equally large cultural and religious differences between those African peoples who were conducting their slave wars and those African peoples who were being enslaved. And there were even greater linguistic and genetic differences between them than there were between the Tatars and the Slavs. By oversimplifying it into 'African-on-African', you're effectively dismissing it as 'Black-on-Black'. I'd be just as entitled to lump all of Europe, the Middle East and North Africa together, and summarize the slave trade as 'Caucasoid-on-Caucasoid'. It doesn't take a genius to realize that things have to be a bit more complex than that.
I don't consider Tartars to be Europeans; they were Turkic peoples from Central Asia foundationally formed by the Golden Horde.
And yes, of course the Africans were culturally different--the various language groups, the different traditions--but Europeans didn't really care about that, of course.
And a) the European states were Christian, b) they had domestic slavery as sanctioned explicitly by the Bible and the Decretum Gratiani, c) they could trade in pagans.
The Catholic Church banned the selling of Christian slaves to Muslims--the main market--multiple times. Slavery was replaced by serfdom everywhere in Europe by the time the Mongols came on the scene.
Christian states nominally tried to avoid selling other Christians; Muslims launched rescue missions and tried to conquer stretches of the Mediterranean coast, or paid ransom (or, the other option which you left out, in spite of the fact that the Christian Europeans did this as well- the Knights of Malta being a notable example- mounted counter-raids of their own to enslave enemy civilians). The Mossi Kingdoms' foreign policy for centuries was one of defeating and conquering the Jihad states, mainly to stop all the slaving.
Christians did not sell white Christians in the time period we're talking about. Turn them into Caribbean slaves as punishment for rebellion? Sure, the Redlegs. Enserf them or bring them close to slavery? Yes. But did Europeans sell other Europeans to a third party of Europeans in the Early Modern Era? No. Portugal and Spain imported Africans as domestic servants instead. You could make a stretch for Poland and Lithuania (there, it ended in 1588) or even Russia, but they had no access to the kind of markets that Portugal and Spain did. Serfdom had replaced slavery.
The Mossi kingdoms had nowhere near the cogent foreign policy or, more importantly, political unity of Muscovy (which became Russia). To compare the two situations is ridiculous.
See how it works both ways? And also, how the emergence of an African Gunpowder Empire or two might serve to change things, if it leads to Sub-Saharan Africa becoming far more uniform religiously, either under Islam or under another native African belief system? Would the African Muslims still be willing to sell other African Muslims on as slaves to pagan Christian Europeans?
The African, Sahelian states of the period did not have the state organization/centralization/longevity that the main gunpowder empires had. Notably, all of said empires were fundamentally touched by Mongol heritage--and built off of Mongol administrative traditions and prior administrative traditions in organizing their states. What does the Sahel have? Kanem-Bornu, sure, and the carousel of various Malian states? It did not have a Perso-Islamic canon, or the Kievan Rus, or something even approachably close to China. Ethiopia, the oldest of them all, had fundamentally changed after the decline of Aksum, the Ethiopian post-Yodit Dark Ages, the Zagwe and then the "Solomonid" revival. For one, it had no sea access.
So, you're saying that North Africa shouldn't be lumped in with the rest of Africa, and that its's a part of the 'Mediterranean world'- but you're also saying that Turks and Circassians don't count as Europeans?
Yes, that's exactly what I'm saying. The successors to the Golden Horde are successors to the steppe tradition and Central Asia, not to Europe. Just because they looked white doesn't make them Europeans.
Genetically, linguistically, culturally, historically- in all of these regards, the Bantu peoples alone (not Sub-Saharan Africans- just the Bantu) were either just as diverse or even more diverse than all of the Indo-European peoples of Eurasia. So if you're lumping all of the peoples of Sub-Saharan Africa together into a single group, you might as well have another group comprising all of the peoples across the entire remainder of Afro-Eurasia together into another single group as well.
In this time period, to European eyes and mercantile purposes, Africa was Africa. The Muslim powers near Europe--the Ottomans especially--knew to differentiate between Eastern and Western Europe, and then between people like the Tatars and, say, the Serbs.
Europeans only differentiated the types of Africans when they needed to make sales comparisons of folks up on the auction block.
No, that's the actual definition of 'The Sahel', which is an African climatological zone.
I'm using it as the cultural term; it is obvious Ethiopia is in the Horn of Africa "zone" as opposed to the Mali-Kanem axis that traditionally defines the Sahel.
And you claim that Gunpowder didn't travel by sea IOTL- so then, how do you explain away the incontrovertible fact that the Majapahit Empire, on the island of Java, got their hands on gunpowder weapons in the early to mid 14th century (at around the same time as, or even earlier than, the Indians started making use of them)? Archeological evidence indicates that either Arab or Indian traders were responsible for introducing gunpowder weapons to the Mahapajit, via the established Indian Ocean commercial trade routes.
They probably got it from China, IMO, although I could definitely have been wrong on the sea-gunpowder thing. In any case, I don't see gunpowder getting from, say, the Swahili or Somali through Ethiopia to the Sahel, not when gunpowder would have just come south from the Arab world.
My POD is simply based on the basic premise that, given that these traders already introduced gunpowder weapons to the respective kingdoms of the Indonesian archipelago, they would have been equally capable of introducing those same gunpowder weapons to the respective kingdoms (and sultanates) along the East Coast of Africa as well, at around the same time (early to mid 13th century)?
I don't think gunpowder got there that early--maybe mid-to-late 14th century.
So, you've already given your own answer to the OP question- in your opinion, it wouldn't have had any real impact, and big Europe would have still smashed puny Africa anyway. Fair enough; everyone's entitled to their own opinion. But IMHO, the most interesting ATLs are the ones which look the most implausible and fantastical at first glance. The key is getting there gradually, making sure that every step along the way is perfectly plausible.
I am a bit of a plausibility zealot, I will admit. You mention Monomotapa; although there was certainly Arabo-Swahili influence, I don't think there was enough sustained contact to give them gunpowder. Hell, in OTL they had gunpowder-users from two ends: the Arab merchants and the Portuguese prazeiros, but gunpowder didn't really catch on. Mutapa was, in any case, not the strongest state; a papally-recognized Kongo is a good bet, if only because the Papal recognition might be able to prevent the worst Portuguese abuses.
I could see Kanem becoming a gunpowder-esque state in the Early Modern Era, seeing as it wouldn't get "Songhai'd"; the spectre of Fulani jihads makes the creation of a centralized or modern state even harder, since nomads tend to make a mockery of the plans of settled folks.
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And considering what became of the Qing and Mughals, being a gunpowder empire isn't exactly a safe protection from the depredations of the trans-oceanic merchants of the European bourgeoisie.