Although Mohammed bin Salman's promotion to crown prince had always been expected among those who follow the royal family closely, the timing was a surprise, and puts the kingdom's future in relatively untested hands.
"The elevation of Mohammad Bin Salman to the Crown Prince will likely mean minimal change to oil production policy but could bring more aggressive foreign policy moves that bring back the political risk premium", RBC Capital Markets said a research note, as quoted by Reuters. He retains defence, oil and other portfolios.
The previous crown prince, Prince Mohammed bin Nayef, the king's nephew and a counter-terrorism chief admired in Washington for putting down an Al-Qaeda campaign of bombings in 2003 to 2006, was relieved of all his posts, according to the decree.
USA -backed forces fighting in Syria are also in close proximity with Iranian-backed forces supporting Syrian President Bashar al-Assad.
Even when there is disagreement, the royal family has long followed a tradition of speaking with one voice, particularly on issues of succession, in order to appear united in front of Saudi Arabia's many tribes and communities.
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The Allegiance Council is a body made up of the sons and prominent grandsons of the founder of the Saudi state, the late King Abdul-Aziz, who vote to pick the king and crown prince from among themselves.
"The president and the crown prince committed to close cooperation to advance our shared goals of security, stability, and prosperity across the Middle East and beyond", said a White House readout.
Regarded warily by some Saudis and by many foreigners as an unknown quantity in the Middle East's traditional status quo power, Prince Mohammed has over the past year set about building his profile with interviews in some Western media.
"The two leaders discussed the priority of cutting off all support for terrorists and extremists, as well as how to resolve the ongoing dispute with Qatar", the statement added.
These include the relations with Qatar, a military intervention that has continued for more than two years in Yemen, an economy adjusting to the loss of oil revenue, and attempts at social reform in a deeply conservative Islamic nation.
"At the same time that Donald Trump gives Saudi Arabia a green light on leadership in the region, he is being attacked by political forces in the USA".
Mohammed bin Salman, who will take on Nayef's old job as interior minister, was relatively unknown in the kingdom when his father came to power 2 1/2 years ago.