Armoured vehicles streamed across the causeway separating Saudi Arabia and Bahrain yesterday. Saudi Arabia said its soldiers were part of a Gulf Co-operation Council (GCC) force to guard government installations. That bald statement does not adequately reflect the Saudi regime's fear that its neighbour – to which it is linked by umbilical cord – will accede to demands from its Shia majority for a constitutional monarchy.
The Wahhabi Saudi regime acting as guardian for Bahrain's Sunni monarchy wants to maintain the status quo both at home and in Bahrain.
The Saudis have already suppressed a potential domestic uprising on 11 March with the biggest carrots and sticks available in the Arab world: through money, $37bn for spending on the population; ideologically, by saying that demonstrations are un-Islamic; and by unleashing the security forces on the streets.
And now it will not tolerate the country next door talking about a constitutional monarchy. For the main aim of the Saudi force will be to ensure that the Bahrain royal family survives in its current form – in the minds of the Saudi rulers, real democracy in Bahrain must be stopped at all costs.
We only have to look at what happened in Saudi Arabia itself: When the Saudi rulers were presented with a petition in 2003 by university professors and professionals calling for constitutional monarchy, the petitioners were put in jail. The subject of constitutional monarchy or any form of representative government is taboo in Saudi Arabia.
The reality is that the royal family in Bahrain was never really under threat. People were asking for a constitutional monarchy, and protesters were only radicalised when the authorities used force against them.
Now the stakes are higher and there is fear that there will be more bloody confrontations because the Saudi army is much more brutal than in Bahrain. They are outsiders. After confrontations they can go back across the causeway.
The two countries are so closely linked – and not just by that strategic causeway. There is a Shia majority in Bahrain and in Saudi Arabia's eastern province. The Shia have always been marginalised and never properly incorporated into the kingdom, and with no representation in the government. The fear for Saudi Arabia is that the Shia in Bahrain, and those in its east, will form an alliance with Iran.
The region is also crucial for the United States. Disruption of oil production in the eastern province of Saudi Arabia could damage the world economy.
The irony is that the GCC – which includes Bahrain, Kuwait, Oman, Qatar, the UAE and Saudi Arabia – was established in 1981 after the outbreak of the Iran-Iraq war to protect the states from outside threats. It was never meant to be used for internal repression of its people or for neighbourly repression.
Mai Yamani is a political analyst and the author of several books on Saudi Arabia