You rarely hear about Yemen's arms build up, except for the time a shipment of North Korean SCUD missiles was intercepted at sea. When it was found that the missiles were headed for Yemen, an American ally in the war on terror, the ship was allowed to go on its way. Yemen's allies have tried to convince the Yemeni government to switch spending priorities. Yemen is at the bottom of the world ranking of what countries spend on things like education and health.
No one really noticed, but Yemen has been buying more and more weapons over the last decade. In 2006, it plans to spend $885 million on the military. The big spurt came between 1998 and 2003, when defense spending tripled. Yemen is not awash with oil wealth, and only has a population of some 15 million. Yemen is the second poorest nation in the Arab world, but is number three in terms of the percentage of its income it spends on weapons.
Why the arms build up down there? No one is sure, and the government just says it has to look after its national security. Part of the problem is unruly tribes along the Saudi Arabian border, factionalism within the country, Islamic terrorism, plus long standing border disputes with Saudi Arabia, and a belief, common among many Yemenis, that it's not really fair that Saudi Arabia should have all that oil. For thousands of years, Yemen was the prosperous part of Arabia (because it's the only part that gets any decent rainfall), while what is now Saudi Arabia was a howling wasteland. Yemenis have never had a comfortable relation with the desert nomads to the north, and the oil wealth up there has not improved matters.
Yemen is buying mostly Russian stuff (and has been for decades), or Chinese and North Korean copies. This is done partly because it's easier for government officials to skim some of the money for themselves. The Russians and Chinese know how to speak corrupt.