Can we please move on from the discussion wether we should have a passage between Kongo and the great lakes? Ever since it was suggested it is being held as a scapegoat why east africa shouldn't be expanded.
Frankly, looking at the geography of contemporary Tanzania, Kenya, South-Sudan, Uganda, Rwanda and Burundi, I cannot see a reasoning for it not to be included, accessible from the Swahili coast. It actually seems very far more passable than other places already on the map, like the Rockys, Persia, the Alps or even contemporary Sudan and Ethiopia. I have only read once why these areas would be implausible to march troops through other than: it just didn't happen. This is the only post going to other detail:
1. Elevation change. Yes you can get over elevation in the Alps or the Himalayas ... but those have a history of well made roads dating back to Roman times (or earlier). The Andes, which historically were crossed by small armies also had Incan & later Spanish roads. There is nothing comparable in this area. Worse, building them, given the equatorial location and climate is going to be much harder.
2. Poor crops. The main traditional crop in the area is sorghum. Crop density is low or absent in the barrier areas and terrible at calorie density. Worse because sorghum is gluten free, it bakes poorly. Carrying enough sorghum just isn't feasible.
3. Irregular ground. Unlike the Sahara, getting out of the Great Lakes region is not a mostly level walk, you have routine scrambling that to this day makes road connections difficult to maintain. The calorie burden for crossing this type of land on foot is quite high.
4. Climate. The highlands (where the people are) are quite nice and hospitable. The surrounding lowlands have an average temperature higher than body temp. This increases the water burden and the calorie burden. This also means that an army on the march needs more water.
5. Water. Due to the geology of the area, a lot of the water in between is alkali. Some of this is straight up non-potable, but even more it has a nasty habit of inhibiting digestion (deactivating some digestive enzymes, increasing stomach acid production, etc.).
6. Animals. Such draft animals as were there during the era were well adapted to the highlands and terrible crossing the waste. Part of this was disease based, but even things like coat density work against armies.
@Jomini: Could you give me references for books (and maybe papers, but science papers are usually behind paywalls, so...) that give more informations to these points (particularly 2, 3, 4 and 5)? Not because I refute your claims but simply out of interest (this discussion here has spiked that considerably).
The thing is, the kingdoms there
could have expanded, they just didn't for far more and different reasons than troop maintenance. Likewise the Europeans/Portugal
could have colonized more than the thin Swahili coast, they just didn't for far more and different reasons than troop maintenance. Reducing everything to whether or not troops could have walked somewhere is extremely one-dimensional.