From 1967 to 1990 two separate Yemeni states existed: the Yemen Arab Republic in the north with its capital at Sana'a, and the People's Democratic Republic of Yemen in the south with its capital at Aden — the only socialist state in the Arab world. On 22 May 1990 they merged into the Republic of Yemen, and two separate nationality systems merged into one.

Because in modern history most map changes run toward separation rather than union — this library has recorded India and Pakistan in 1947, Bangladesh in 1971, Tuvalu and Kiribati in 1974, Bougainville in 2019, and the Korean peninsula in 1945. Two independent, internationally recognised states voluntarily merging happens very seldom.

No. A brief civil war occurred in 1994 between forces of the two former states, and southern dissatisfaction over the division of power and resources remains a factor in Yemeni politics. The lesson: merging two states on paper is far easier than merging two administrations, two security establishments and two ways of allocating resources.

When territory is controlled by different parties, the issuance of identity documents and passports fragments accordingly. Documents issued by different authorities may be recognised differently depending on the country and period; people may have to travel to another area to obtain documents; and those born during the conflict risk not being registered at birth. Nobody was stripped of nationality — there is simply no longer one authority to certify it.

The word mocha comes from the port of Mokha on Yemen's Red Sea coast. The coffee plant originates in the Ethiopian highlands, but Yemen is where coffee was first cultivated systematically and became a traded commodity from around the fifteenth century, and Mokha was the principal export gateway to the world for centuries. Today Yemen accounts for only a very small share of world output but its coffee remains among the most expensive on the market.

Qat is a plant whose leaves contain a stimulant and are widely chewed in Yemen, and it consumes a very large share of the country's irrigation water because it earns more than food crops so farmers switch to it. This is a clear example of an economically rational household choice producing severe national consequences. Sana'a has for years been among the capitals warned of exhausting its extractable groundwater.

The Hadhramaut region of southeastern Yemen has a long seafaring tradition, and for centuries Hadhrami merchant families followed the monsoon to India, East Africa and above all the Malay archipelago. The result is large communities in Indonesia, Malaysia and Singapore of Hadhrami origin. It is a very early example of a transoceanic diaspora, formed centuries before passports or modern nationality existed.

It has among the highest levels of plant endemism in the world, meaning a very large share of its species exist nowhere else on earth. The best known is the dragon's blood tree, its canopy spreading like an inverted umbrella and its resin blood-red. The island was inscribed as a UNESCO World Heritage site in 2008 and is under pressure from climate change and human impact.