The country is trying to prevent "militants" from infiltrating its borders
Diplomats say Saudi Arabia is urgently stepping up border controls after the surge of militancy last year, fueled by weapons smuggled across thousands of kilometers of desert and mountain borders.
Yemen and Saudi Arabia shared one of the longest undefined borders in the world. Only a small part of the line had ever been agreed: a portion at the extreme north-western end stretching from a point just north of Midi on the Red Sea coast to Najran oasis. That was in 1934 under the Treaty of Ta'if, when, after a brief war, two ethnically Yemeni provinces, Asir and Najran, were ceded to the Saudis. The remaining eastern portion of the frontier - totally undefined - ran for almost 1,000 miles through mountains and desert, mostly unpopulated, on the fringes of the Empty Quarter. To the west, the maritime border in the Red Sea was also undefined, further hampering oil exploration.
Northern Yemenis were allowed to enter the kingdom on terms which were easier than those for nationals of other countries (including the PDRY). They had no need for a Saudi sponsor, and were allowed to own businesses without the customary Saudi partner. In Sana'a's view these privileges were not merely a favour bestowed by the Saudis but had a legal basis: letters exchanged by the Saudi and Yemeni leaders in 1934 at the signing of the Treaty of Ta'if (which delineated part of the common border) can be interpreted as allowing relatively unrestricted Yemeni entry into the kingdom4. Naturally, Sana'a made a point of interpreting them in this way and regarded them as an integral part of the treaty.