The fact that Abu Dhabi's government-owned Mubadala Development Company could open its vaults and hand over $1.35 billion for 7.5 percent of the U.S.-based Carlyle Group may be newsworthy, but it's not surprising. What may be more surprising is that Mubadala didn't spring for more.

Simply put, Mubadala is the cash camel of the United Arab Emirates, one of the wealthiest of the oil-and-natural-gas-producing members of the 12-nation Organization of Petroleum Exporting Countries.

In the Gulf, there is a saying: Dubai is the emirate with all the flash, but Abu Dhabi—the biggest and richest of the U.A.E.'s seven sheikhdoms—has the cash.

Need more evidence? Look no further than today's headlines: State-owned Abu Dhabi National Energy Co. agreed to buy PrimeWest Energy Trust of Canada for $5 billion in cash and assumed debt. It is the biggest-ever North American takeover by a United Arab Emirates company.

It has the cash because virtually all of the U.A.E.'s crude oil—10 billion barrels of it, representing almost 10 percent of the world's total of proven reserves—and 5.8 billion cubic meters of natural gas lie in Abu Dhabi.

Nearly 92 percent of the country's gas reserves—4 percent of the world's total—are in Abu Dhabi itself and in the Khuff reservoir beneath the offshore oil fields of Umm Shaif and Abu al-Bukhoosh. It ranks among the largest single gas reservoirs in the world.

At the current production rate of some 2 million barrels a day, estimates suggest the U.A.E.'s reserves could last another 150 years. And if oil prices keep hovering in the range of this week's record $80 a barrel, the U.A.E.'s budget surpluses will grow to a point where it just may run out of places in which to invest.

Under an agreement forged by the late Sheik Zayed Al Nahayan, when the U.A.E. was created in 1971 from the British possession known as the Trucial States, the other six emirates received the equivalent of monthly allowances. Zayed is widely credited with holding together the seven emirates by keeping various sheiks from feuding openly over the country's natural wealth.

How much disposable cash does Abu Dhabi possess? Conservatively speaking, its surplus for 2007 is likely to be in excess of $500 billion. Abu Dhabi has already invested more than $1 trillion abroad, some of it in U.S. Treasury bills.

This year, the six countries comprising the Gulf Cooperation Council—Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the United Arab Emirates—are expected to invest nearly $70 billion overseas; in 2006, they invested about half that figure. Just last week, for example, the U.A.E.'s bitter rival, neighboring Qatar, announced it was taking a 20 percent stake in the London Stock Exchange.

As for Mubadala, its existence highlights a fundamental premise of modern-day nation building: A country's wealth, especially accruing from its natural resources, must be put in service of economic growth. And in this age of galloping globalization, it isn't enough simply to invest. It is important to invest wisely. It is equally important to invest widely.

That is what Mubadala has been doing since its creation five years ago as a wholly owned unit of the government of the emirate of Abu Dhabi. Its objectives are manifold, but they all underscore a central thesis: The emirate needs to broaden its economy and embrace a variety of enterprises.

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