Rising conflict between Sunni Saudi Arabia and Shiite Iran, two of the most significant powers in the Middle East, has potentially far-reaching consequences for the entire region.
In which countries will the conflict have greatest impact?
The first to be affected are Syria and Yemen, where both Riyadh and Tehran are major players in the respective civil wars. There have also been protests by Shiites against the Saudi monarchy in the Gulf kingdom of Bahrain and in Iraq.
Why is Saudi Arabia allowing the conflict to escalate?
Despite its immense oil reserves, Saudi Arabia is acting from a position of weakness in its aim of countering Iran's influence on the Arab world. The kingdom sees itself as under threat from Tehran. The Saudi government believes this perception has been confirmed by the deal on Iran's nuclear programme.
The Islamic State terrorist militia, which has advanced to the frontiers of Saudi Arabia, is also applying pressure to the monarchy. At the same time, Riyadh faces a gaping hole amounting to billions of dollars in its budget as a result of the collapse in the price of oil.
What interests is Iran pursuing in the Arab world?
Tehran regards itself as the most important country of the region strategically and is bidding for a say in all regional developments. It aims to make clear to its Arab neighbours that Islam forms a political ideology to be implemented independently of the West - and particularly of the United States.
This ideology includes the central doctrine of Iranian foreign policy - the liberation of Palestine from Israeli occupation and the founding of a Palestinian state. Tehran also aims to offer support to Shiite minority movements in Arab countries with Sunni governments.
What are the implications for the Syrian peace talks?
Iran is one of the Syrian regime's most important allies, while Saudi Arabia backs the rebels. Ending the bloody conflict would be impossible without the cooperation of these two arch-rivals. Only if Iran withdraws its support can the regime of Syrian President Bashar al-Assad be toppled - and this is a precondition for an end to the conflict for opponents of the Syrian government.
Last year, the respective positions of Saudi Arabia and Iran appeared to be approaching each other during negotiations on a political roadmap to end the civil war. They agreed on talks between the Syrian regime and its opponents. These talks were set to begin on January 25, but this date could now be in doubt.
What effects do the current tensions have on the civil war in Yemen?
The situation there is similar to that in Syria. Saudi Arabia provides military support to the government of President Abd Rabu Mansour Hadi, whereas Iran backs the Shiite Houthi rebels, who have seized control of large swathes of the country. Peace talks in Switzerland in December ended without result, and a renewed ceasefire also broke down. These talks are to be resumed in January, but whether this will happen is now in doubt.
Why are there protests in Bahrain?
Although this Gulf kingdom has a Sunni ruler, the majority of the population is Shiite. During the Arab Spring uprisings in 2011, Shiite protesters took to the streets to demonstrate against the monarchy, which used force to suppress the protests. In this it was supported by Saudi Arabia, which sent troops across the border into Bahrain.
The crisis in the island state on the western shores of the Gulf has not gone away, as Bahrain continues to vigorously suppress the Shiite opposition. Saudi Arabia fears that the unrest could spread to its own Shiite minority, which is concentrated nearby in the east of the country.