Security has deteriorated on the Peninsula since the Arab Spring. Yemen remains an Al Qaeda stronghold.
From the BBC:
Border security has dramatically worsened in the aftermath of the revolution, as thousands of illegal immigrants, drug smugglers and gun runners try to slip from impoverished Yemen into Saudi Arabia, one of the world's richest countries, Lt al-Ahmari told the BBC's Frank Gardner.Will there be United nations resolutions condemning this apartheid wall? Will the International Solidarity Movement send people to sabotage it? Will there be demonstrations in front of Saudi embassies and consulates throughout the world?
Five Saudi border guards had recently been killed along the border in shoot-outs with well-armed smugglers, he added.
The first part of the fence has already been built on the coast, slowing down - but not stopping - the tide of illegal immigrants.
Smugglers and militants have already shown ingenuity to circumvent the new border by using women, mules and secret paths to get into Saudi Arabia..
We already know that answer. All the nations of the world have not only the right but the obligation to protect their citizens. All nations but Israel.