It is the least known corner of modern Arabia, but was a place of legend in the ancient world - Arabia Felix, so named for its rich fertility. The verdant slopes of its mountains are still covered in the bushes that produce qat, the cocaine- like leaf chewed by Yemenis. It also produced spices, incense and myrrh, and traded with India and Europe, creating an image of wealth and luxury.
Economic decay pushed the two modern states into unity in 1990. They had long been at daggers drawn, and border skirmishes were frequent. But the discovery of oil, much of it located in the border area, spurred them to bury their differences. It was hoped this would reverse the underdevelopment that had made the north one of the world's poorest states, and which hit the south, a satellite of Moscow, after the collapse of the Soviet Union.
It is a five-hour sweaty and back-breaking drive through the mountains from Sanaa, capital of the north, to Aden, its southern counterpart, but in mental terms one might as well add another 500 years.
Sanaa is a medieval village, its old centre crowded with the extravagantly decorated multi- storey mud tower houses characteristic of old Yemen. Aden, by contrast, bears the mark of a city that has survived British and Soviet imperialism, its once-elegant hotels now fly-ridden and decaying.
Their histories, though different, are both bloody. South Yemen won its independence from Britain in 1967 after a prolonged insurgency; then it became a Marxist regime, which splintered in 1986 after a coup devastated large parts of Aden. North Yemen, part of the Ottoman empire until 1919, emerged from the Middle Ages only after a bloody conflict in the 1960s, in which Egypt backed the republicans and Saudi Arabia and Britain the royalists.
Much hope was invested in unification. Yemen has a relatively free press and many of the nascent institutions of civil society. Last year, it held the first honest elections on the Arabian peninsula. But the results only intensified conflict between the political elites of north and south.
The northern and southern armies were never unified. President Ali Abdullah Saleh's northern General People's Congress and Vice-President Ali Salem al-Baidh, leader of the southern Yemen Socialist Party, have been at loggerheads for four years. Last week, this erupted into conflict between the two armies.
Diplomats said yesterday that the the northerners aimed to remove al-Baidh and impose their rule on the south. 'The military manoeuvrings are an attempt to put pressure on the southern leadership to remove him,' said the diplomat. Although the fighting was still short of civil war, it could rapidly descend into it, he warned.
The struggle is complicated by the country's social structure. Yemen is a land of tribes, whose alliances and enmities render a purely north-south conflict meaningless: al-Baidh has the support of one northern tribal confederation, while another backs the north. Imagine Afghanistan mixed with Somalia, and you have an idea of the potential for conflict.
The tribes control the hills, their Toyota Landcruisers decorated with machine-guns and anti-tank weapons, their shoulders bearing Kalashnikovs. It is probably the most heavily armed country in the world: 50 million weapons among a population of 13 million; some tribal chiefs even boast tanks and heavy artillery.
The north has little support from states in the region, which fear fragmentation. Much depends on the attitude of Saudi Arabia. Key figures in the royal family are reported to be furious with the north.
Yet Saudi Arabia has also been fomenting conflict in Yemen. Its longer-term ambition is to prevent a single powerful state emerging on its southern border. 'They want a continuing state of low-level instability,' said a Western analyst.
'Going back to two states is probably not an option,' said a diplomat yesterday. 'There could be half-a-dozen small warring states.' That is an option no side wants, but it may yet be the result.