This thread might have the answers you're looking for courtesy of myself and /u/lightingbolt50

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I'll copy paste what I had:

I'm not familiar enough with the collapse of the Somali state in 1991 in enough detail to really explain what happened there, but I can at least give you some background up to what really seemed to be the start of Somalia's long decline.

It dates back to the immediate post-WW2 period, where the Allies (primarily British and Americans) were reorganizing the postwar Horn of Africa. Italy's colonies in the region were to be transferred to the victors, but there was debate on how that was to be done. Key issues revolved around Ethiopia - Emperor Haile Selaissie had his eyes on pretty much the entire region, while the British were looking at making Eritrea independent and breaking off the predominantly Somali Ogaden region of Ethiopia and awarding it to the Somali colonies. That ended up going largely in Selaissie's favor, however. Eritrea was given to Ethiopia in return for a (soon to be broken) promise that it would be ruled as an autonomous democratic region, and, while Selassie didn't manage to convince the Allies to give him control of all of Somalia, he did manage to retain Ethiopia's pre-war borders with respect to Somalia.

Soon after this, Britain had begun the process of releasing its African colonies, Somalia among them. In Somalia, British policy as part of the decolonization process was to try to forge a Somali identity by pushing Somali nationalism with the hopes of pulling the population away from tribalism, which traditionally has dominated Somali politics. Unfortunately, there's large populations of Somalis living abroad in neighboring Djibouti, Ethiopia, and Kenya, and the nationalism Britain tried to foster inevitably led to irredentism (albeit not without blame, given oppressive policies by the Ethiopian government and some suspect elections in Djibouti and Somali regions of Kenya). Ogaden became a major point of contention, with Somali forces clashing with Ethiopian forces in 1964 and an insurgency starting up.

Things became more unstable once Siad Barre took over Somalia in 1969. He leveraged Soviet support to bolster his position, and tried to deflect the endemic tribalism and general unrest by fostering Somali irredentism. The army was equipped with massive amounts of modern Soviet equipment, and Barre allowed the Soviets to construct a naval base at Berbera, on Somalia's north coast. He provided extensive support for Somali separatist groups abroad, particularly the West Somali Liberation Front (WSLF) in Ogaden. Tensions increased over the years, while at the same time the situation in Ethiopia deteriorated. Emperor Selassie became increasingly incapable and ultimately in 1974 suffered a revolution under similar circumstances to the 1979 Iranian Revolution. He was overthrown, and a communist regime (the Derg) took his place with all the purges and human rights abuses typically accompanying such developments.

Ethiopia's turn towards communism and the chaos that came with it led to the US dropping its support for the country, while the Soviets saw an opportunity to move in and get the whole Horn under their wing. But in 1978, as the Soviets and Cubans were working on smoothing Ethiopian-Somali relations, Barre suddenly invaded Ethiopia without warning. Nearly the entire Ogaden region was overrun before Somali forces lost their steam, while the Soviets and Cubans tried to pressure Barre to withdraw (largely by withholding military aid). Barre, in desperation as the war ground down, believed he could get the support of the Chinese or Americans if he were to drop the Soviets, so he made the decision to eject the Soviets (several tens of thousands of them) from the country - even the brand new naval base they'd just completed at Berbera.

And that's where things started to go downhill. The Soviets responded to the slight by launching their largest strategic airlift operation since WW2. They airlifted as much men and equipment as they could into the Ethiopian capital, shipped in heavy equipment that couldn't be flown in, and even diverted a satellite launch to provide timely intelligence. Aircraft were diverted from Yemen, 50,000 Cubans and several thousand Yemenis were flown in, and the Ethiopans were mostly rearmed and retrained in a matter of months. Meanwhile, Somalia found that President Carter wasn't too keen on propping up a dictatorial regime in the middle of a blatant war of aggression that had just shown a willingness to flip sides at a whim for short-term gain, and the Chinese, while more willing to supply Somalia, weren't willing or able to provide the kind of timely support Barre was looking for. The Somali Army and Air Force were annihilated soon after, and, although the US nominally stepped in to back Siad Barre, Somalia never received the kind of backing that they had under the Soviets - rather, US support was to prevent Ethiopia from dominating the region.

And that's where my knowledge of the situation largely drops off. The collapse of Somalia can mostly be traced to these events at their core, as the devastation of the war and cutoff of aid hurt Somalia's economy and reignited tribal tensions, and the remainder of the Cold War was marked by a slow decline superficially similar to that of Ethiopia until various rebel groups finally ousted Siad Barre in 1991. Unlike Ethiopia, however, where the rebel groups formed a coalition largely led by the TPLF, the Somali rebels weren't so cooperative, so the situation devolved into a failed state.