ABU DHABI (NYTIMES) - Prince Mohammed bin Zayed, the 29-year-old commander of the almost negligible air force of the United Arab Emirates, had come to Washington shopping for weapons.

It was 1991, in the months after Iraq's invasion of Kuwait, and the young prince wanted to buy so much military hardware to protect his own oil-rich monarchy - from Hellfire missiles to Apache helicopters to F-16 jets - that Congress worried he might destabilise the region.

But the Pentagon, trying to cultivate accommodating allies in the Gulf, had identified Prince Mohammed as a promising partner. The favourite son of the illiterate Bedouin who founded the UAE, Prince Mohammed was a serious-minded, British-trained helicopter pilot who had persuaded his father to transfer US$4 billion to the US Treasury to help pay for the 1991 war in Iraq.

Mr Richard Clarke, then an assistant secretary of state, reassured lawmakers that the young prince would never become an aggressor.

"The UAE is not now and never will be a threat to stability or peace in the region," Mr Clarke said in congressional testimony. "That is very hard to imagine. Indeed, the UAE is a force for peace."

Thirty years later, Prince Mohammed, now 58, crown prince of Abu Dhabi and de facto ruler of the UAE, is arguably the most powerful leader in the Arab world. He is also among the most influential foreign voices in Washington, urging the United States to adopt his increasingly bellicose approach to the region.

Prince Mohammed is almost unknown to the American public, and his tiny country has fewer citizens than Rhode Island has. But he may be the richest man in the world.

He controls sovereign wealth funds worth US$1.3 trillion (S$1.8 trillion), more than any other country. His influence operation in Washington is legendary. His military is the Arab world's most potent, equipped through its work with the US to conduct high-tech surveillance and combat operations far beyond its borders.

For decades, the prince has been a key US ally, following Washington's lead, but now he is going his own way. His special forces are active in Yemen, Libya, Somalia and Egypt's North Sinai. He has worked to thwart democratic transitions in the Middle East, helped install a reliable autocrat in Egypt and boosted a protege to power in Saudi Arabia.

At times, the prince has contradicted US policy and destabilised neighbours. Rights groups have criticised him for jailing dissidents at home, for his role in creating a humanitarian crisis in Yemen, and for backing the Saudi prince whose agents killed dissident writer Jamal Khashoggi.

Yet under the Trump administration, his influence in Washington appears greater than ever. He has a rapport with President Donald Trump, who has frequently adopted the prince's views on Qatar, Libya and Saudi Arabia, even over the advice of Cabinet officials or senior national security staff.

Western diplomats who know the prince - known as MBZ - say he is obsessed with two enemies, Iran and the Muslim Brotherhood. Mr Trump has sought to move strongly against both and last week took steps to bypass congressional opposition to keep selling weapons to both Saudi Arabia and the UAE.

"MBZ has an extraordinary way of telling Americans his own interests but making it come across as good advice about the region," said Mr Ben Rhodes, a deputy national security adviser under former president Barack Obama, whose sympathy for the Arab Spring and negotiations with Iran brought blistering criticism from the Emirati prince.

When it comes to influence in Washington, Mr Rhodes added: "MBZ is in a class by himself."

Prince Mohammed worked assiduously before the presidential election to crack Mr Trump's inner circle, and secured a secret meeting during the transition with the president's son-in-law, Mr Jared Kushner. The prince also tried to broker talks between the Trump administration and Russia, a gambit that later entangled him in the special counsel's investigation into foreign election interference.

Today, at least five people working for Prince Mohammed have been caught up in criminal investigations growing out of that inquiry. A regular visitor to the US for three decades, Prince Mohammed has now stayed away for two years, in part because he fears prosecutors might seek to question him or his aides, according to two people familiar with this thinking.

The UAE Embassy in Washington declined to comment.

The prince's many American defenders say it is only prudent of him to try to shape US policy, as many governments do, and that he sees his interventions as an attempt to compensate for an American pullback.

But Prince Mohammed's critics say his rise is a study in unintended consequences. The obscure young prince whom Washington adopted as a pliant ally is now fanning his volatile region's flames.

By arming the UAE with such advanced surveillance technology, commandos and weaponry, argued Ms Tamara Cofman Wittes, a former State Department official and fellow at the Brookings Institution, "we have created a little Frankenstein".

The UAE is a tiny federation of city-states, yet Abu Dhabi accounts for 6 per cent of the world's proven oil reserves, making it a tempting target to a larger neighbour like Iran.

The UAE began allowing US forces to operate from bases inside the country during the Persian Gulf War of 1991. Since then, the Prince's commandos and air forces have been deployed with the Americans in Kosovo, Somalia, Afghanistan and Libya, as well as against the Islamic State in Iraq and Syria group.

He has recruited US commanders to run his military and former spies to set up his intelligence services. He also acquired more weaponry in the four years before 2010 than the other five Gulf monarchies combined.

With advice from former top military commanders including former defence secretary James Mattis, Prince Mohammed has even developed an Emirati defence industry, producing an amphibious armoured vehicle known as The Beast and others that he is supplying to clients in Libya and Egypt. The Emiratis are also preparing a low-altitude propeller-driven bomber for counterinsurgency combat, an idea Mr Mattis had long recommended for the US, a former officer close to him said.

Prince Mohammed has often told US officials that he saw Israel as an ally against Iran and the Muslim Brotherhood. Israel trusted him enough to sell him upgrades for his F-16s, as well as advanced mobile phone spyware.

To many in Washington, Prince Mohammed had become America's best friend in the region, a dutiful partner who could be counted on for tasks from countering Iranian influence in Lebanon to funding construction in Iraq.

"It was well known that if you needed something done in the Middle East, the Emiratis would do it," recalled Mr Richard Olson, a former US ambassador to Abu Dhabi.

When he meets Americans, Prince Mohammed emphasises the things that make the UAE more liberal than their neighbours. Women have more opportunities: a third of the Cabinet ministers are female. Unlike Saudi Arabia, the UAE allows Christian churches and Hindu or Sikh temples, partly to accommodate a vast foreign workforce. (The country is estimated to have nine million residents, but fewer than a million citizens; the rest are foreign workers.)

To underscore the point, the Prince last year created a Ministry of Tolerance and declared this the "Year of Tolerance". He has hosted the Special Olympics and Pope Francis.

Prince Mohammed seemed to find a kindred spirit when president Barack Obama took office in 2009, White House aides said. Both were detached, analytic and intrigued by big questions. For a time, Mr Obama sought out phone conversations with Prince Mohammed more than with any other foreign leader, several senior White House officials recalled.

But the Arab Spring came between them. Uprisings swept the region. The Muslim Brotherhood was winning elections. And Mr Obama appeared to endorse the demands for democracy - though in Syria, where the uprising threatened a foe of the Emiratis, he balked at military action.

Then it emerged that the Obama administration was in secret nuclear talks with Iran.

"They felt not only ignored - they felt betrayed by the Obama administration, and I think Prince Mohammed felt it particularly and personally," said Mr Stephen Hadley, a national security adviser under former president George W. Bush who has stayed close to the prince.

After the uprisings, Prince Mohammed saw the United Arab Emirates as the only one of the 22 Arab states still on its feet, with a stable government, functional economy, able military and "moderate ideology", said Emirati political scientist Abdulkhalleq Abdulla with access to the country's rulers.

"The UAE is part of this very dangerous region that is getting more dangerous by the day - full of chaos and wars and extremists," he said. "So the motivation is this: If we don't go after the bad guys, they will come after us."

Mr Trump has repeatedly backed the positions of the Emirati prince: By endorsing his Saudi protege, Crown Prince Mohammed bin Salman, after the Khashoggi killing, by applauding the isolation of Qatar even as the secretaries of state and defence publicly opposed it, by cancelling the nuclear deal with Iran, by seeking to designate the Muslim Brotherhood a terrorist group, and by vetoing legislation to cut off US military support for Saudi and Emirati forces in Yemen.