It sounds like you're referring to Eritrea's independence in 1991, and there's a lot of context that needs to be added to explain that.

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Eritrea in its current form draws its origins to Italy's colony in the Red Sea. Granted, there are some ethnic similarities between the Tigrinya and Tigre peoples that form the bulk of Eritrea's population and the Tigrayans just across the border in Ethiopia that date back to the Kingdom of Aksum, the groups became more distinct over the centuries as they came under the dominance of different polities in the region. In recent history, the Italian occupation and administration led to a significant divide between it and the rest of Ethiopia - not necessarily due to any "Italianization" of the colony, but more due to the result of the post-WW2 disposition of Italy's colonies.

After WW2, Britain had been pushing for a reorganization of the Horn of Africa along more ethnic lines. Ethiopia was largely to be restored to its pre-1935 state, but the eastern Somali-dominated region of Ogaden would be broken off into a united Somali state and Eritrea would become an independent state. Haile Selassie, however, had imperialist ambitions that were backed up by US support. Selassie wanted all of the Italian colonies - both Eritrea and Somalia.

Ultimately, the UN would award Eritrea to Selassie in 1952 with Resolution 390 (A). This resolution placed Eritrea in a federation with Ethiopia for a period of ten years, during which Eritrea would be ruled separately from Ethiopia with several rights and privileges spelled out, such as the right to democratic elections. However, Selassie never seemed to have any intention of honoring this agreement. As the ten years went on, the rights defined by Resolution 390 (A) were slowly eroded until 1962, when Haile Selassie unilaterally incorporated Eritrea as a regular province of Ethiopia.

At that point, a rebel group had sprung up in the form the Eritrean Liberation Front (ELF). The ELF failed to gain much ground during Selassie's years, and would eventually be overshadowed and sidelined by the Eritrean People's Liberation Front that splintered off from it in 1970. When the Ethiopian Revolution happened in 1975, it let the floodgates loose for dissent not just in Eritrea, but among the diverse population of Ethiopia as well. The worsening situation in Ethiopia due to the myriad rebel groups and Somali invasion in 1977 gave the EPLF some breathing space, and even after the Soviets and Cubans arrived to stabilize the Ethiopian regime, they proved unable to suppress the rebels across the country.

During this period, the Tigray People's Liberation Front would form just across the border in Ethiopia. Straddling the main lines of communication between Eritrea and Ethiopia proper, Tigray province proved crucial to the Ethiopian Regime's control of Eritrea, and thus it was clearly in the interests of the EPLF and TPLF to cooperate. This cooperation would allow for decisive victories by both rebel groups against government forces and provide a basis for cooperation that would prove crucial in peace talks.

Ultimately, the TPLF would be one of the leading members of the EPRDF coalition that would overthrow the Ethiopian regime in 1991, while the EPLF in Eritrea had largely drove out the Ethiopians around the same time. Peace talks were established by the US, which included provisions for Eritrea to vote on independence. Eritreans voted overwhelmingly in favor of independence, which was declared in 1993. Unfortunately, relations soured between Eritrea and Ethiopia in the years that followed, leading to the outbreak of war in 1998 and the occasional clashes ever since over minor border disputes.

Fortunately for Ethiopia, they've been able to rely on their past methods for dealing with being land-locked. Back in 1897, in the aftermath of the First Italo-Ethiopian War, they were in a similar situation - a hostile neighbor kept them landlocked practically within sight of the sea. But a friendly colonial power - France - controlled Djibouti, and they had every reason to want to keep Ethiopia out of Italy's hands. Thus, a railway was constructed by way of Dire Dawa to Djibouti, providing Ethiopia access to the sea. This has historically served as Ethiopia's main artery for seaborne trade. The railway fell into disrepair during the Ethiopian Civil War, but in recent years it has been refurbished. Though nominally independent, Djibouti remains willing to provide Ethiopia with access to the sea through this rail link.