I read dozens of different articles over the time; don't think that any of these are available online, so can't really help with sources of reference.

But, from recollection, my understanding is that there was one 'underlying factor' that was brewing not only before Somalia gained independence or Barre came to power, but eventually resulted in destruction of Somalia as a state.

Somalia never existed in its 'modern-day' form - as an independent and united country - before former British Somaliland and Italian Somaliland were released into independence as a unified parliamentary republic, in 1960.

Immediately after, the government came under severe pressure from multiple political parties to 'establish a union of all areas predominantly populated by the Somalis' - i.e. large parts of Ethiopia, Kenya, and what was then Affars and Issas (meanwhile Djibouti). Means: the country has got something like a 'historic mission' right from the start - and this was mirrored in the national flag too: this consisted of five starts, two representing the former British and Italian colonies and the three others the part of 'greater Somali nation'.

Correspondingly, already the government of the first Somali president, Aden Abdullah Osman Daar, not only refused to acknowledge several related treaties (like the Anglo-Ethiopian treaty recognizing Ethiopian claim over the Ogaden, from 1954), but began searching for ways to develop a big and powerful military.

Daar requested help from the USA and Italy, but both rejected, and thus - and as so many others in similar position (see Egyptian president Nasser) - ended in the USSR instead. In 1963, Moscow granted an 'unconditional' loan of US$ 32 million with a fixed 20-year repayment schedule to Somalia: most of this was spent for acquisition of fighter jets, tanks and training of about 14,000 Somali personnel. That loan was subsequently increased to US$ 65 million - partially paid back by granting basing rights for the port of Berbera to Soviets - much of which was spent to construct various military bases.

Even then, many Somalis found this 'insufficient' and 'too slow', and Daar was defeated in elections of 1967. By that time, the issue of Ogaden was already a 'hot' topic. In 1964 for example, several border incidents occurred but these only made it clear that the Somali military was still not in condition to fight a war with its neighbours. This was the reason for next Somali president - Abdirashid Ali Shermarke - borrowing more money from Moscow, and buying more arms, in 1969. Even then, this was not satisfactory for the military: this launched a coup on 15 October of that year, assassinated Shermarke and established the Supreme Revolutionary Council presided by Major-General Siad Barre in power.

Barre immediately announced that his government would support, 'all liberation movements in countries under colonial rule', and those 'in illegally occupied territories'. This promise of re-union of all Somalis is what he used to maintain himself in power for the next 20 years: while composed of members from multiple Somali tribes, military and security apparatus were held together by the dream of greater Somalia. I.e. Barre's regime remained in power thanks to support from leaders of all the major Somali tribes - and as long as he could maintain hope that these dreams would be realized sometimes in the future, everything was fine.

Because Somalia was still largely reliant on development aid provided from abroad and could not continue its military build-up on its own, Barre was forced to continue the policy of borrowing money from Moscow and buying additional arms. Contrary to earlier times, when they did not meddle into Somali politics, in early 1970s Soviets conditioned their aid with Barre declaring Somalia a 'socialist' and 'democratic republic'. This was clearly a farce, but important to the USSR within the context of the Cold War.

Said - done: Barre declared Somalia a 'Socialist Democratic Republic', but this only created the impression of a 'developing socialist country' in the West. In turn, during the following years, Berbera was developed into a major Soviet base in Africa, while Somalia received large batches of MiGs and Soviet tanks. This would probably go on like that for a number of years longer, wasn't there a severe drought of 1974-1975 that had negative repercussions on all aspects of public life in the country. Mogadishu simply couldn't pay back during that time.

Nevertheless, in 1974 Ethiopian Emperor Selassie was overthrown, and subsequently that country was thrown into chaos by bloody infighting between various power cliques. This in turn provided an opportunity for Somalis to attempt realizing their biggest dream - namely 'recovery' of the Ogaden. That's why they invaded in 1977.

Early on Somalis conquered most of Ogaden, but then Barre made a critical mistake. Namely, Moscow was curious to effect something like a union of Somalia, Ethiopia and South Yemen, not to support a war between Ethiopia and Somalia. Plus, the new Ethiopian strongman, Heile Mariam, proved an excellent negotiator on the international scene: he befriended Fidel Castro and - via Cuba - established ties to the USSR. After lengthy negotiations, he declared Ethiopia a 'socialist' and 'democratic republic' in order to ascertain deliveries of Soviet arms. Barre flipped out over this issue and literally kicked all Soviets out of Somalia, in November 1977. Moscow reacted with a 'revenge' in form of immense deliveries of arms to Ethiopia - in turn making that country capable of defeating Somali invasion and forcing Somali military out of Ogaden.

Barre subsequently turned to Italy and China for aid, but over the time it became clear that his military could not recover under his rule any more; i.e. that the country could not accomplish its 'historic mission' - at least not under Barre. This realisation created doubts over support for his rule, this then created first rifts between him and various tribes, and these led to more rifts - all of which eventually resulted in Barre's downfall of 1989 and the subsequent anarchy.

Thus, from my POV, the 'underlying factor' - the idea of unification of all the land predominantly populated by Somalis - was what helped keep Somalia together, it was what helped it enjoy relative development of the 1960s and early 1970s, but also what helped destroy it in the 1980s.