(Adds analysts, Pentagon, background)
By Yara Bayoumy and Jonathan Landay
WASHINGTON, Nov 6 (Reuters) - U.S. President Donald Trump on Monday endorsed a move by Saudi Arabia’s future king that tightened his grip on power through the arrests of royals, ministers and investors in an anti-corruption purge.
The endorsement cemented a U.S.-Saudi relationship that has improved dramatically under Trump's presidency, partly because of both leaders' vision of confronting Riyadh's arch-rival Iran more aggressively in the region.
Trump tweeted on Monday that he had "great confidence in King Salman and the Crown Prince of Saudi Arabia" following the mass arrests - the biggest such purge of the kingdom's affluent elite in its modern history.
Trump also tweeted that "they know exactly what they are doing," adding: "Some of those they are harshly treating have been 'milking' their country for years."
The purge was the latest in a series of dramatic steps by Crown Prince Mohammed bin Salman, or MbS as he is commonly referred to in Western circles, to assert Saudi influence internationally and amass more power for himself at home.
A U.S. official who declined to be named told Reuters that MbS "has become the primary driver of Saudi policy-making. He has moved aggressively to sideline opponents, concentrate decision-making authority, and establish himself as the undisputed heir to the al Saud legacy."
"He seeks to reinvigorate the public’s confidence in the Saudi monarchy by diversifying the economy, loosening religious restrictions, and carrying out wide ranging social reforms,” the official said.
'IMPRESSIVE FEAT'
Trump's son-in-law, Jared Kushner, the president's senior adviser, who has cultivated a close relationship with MbS, recently returned from Saudi Arabia, fuelling speculation on whether he may have had wind of MbS' plans.
The White House said at the time the trip was reported that it was within the context of Kushner's efforts on Israeli-Palestinian peace conversations.
Among those arrested were billionaire investor Alwaleed bin Talal, who is one of the kingdom’s most prominent businessmen and whose investments in companies like Twitter make him the most recognized Saudi name on Wall Street.
"It's an impressive feat to have basically neutered virtually all of the sources of potential opposition, dissent rivalry, whether it's religious, media, political, military," said Rob Malley, vice president for policy of the International Crisis Group.
"On the other hand... it's always risky to make too many enemies at the same time. It's not clear how those enemies can react," said Malley, a former senior adviser on Middle East affairs under President Barack Obama.