DOHA, Qatar — Among senior U.S. officials, the wealthy Persian Gulf nation of the United Arab Emirates is known by an affectionate nickname: Little Sparta. Subscribe to read this story ad-free Get unlimited access to ad-free articles and exclusive content. The moniker, often attributed to former U.S. Defense Secretary James Mattis, reflects the American admiration for the UAE’s investment in its military and its willingness to act decisively, compared to its slower-moving Gulf neighbors. This week, “Little Sparta” lived up to its reputation for pivoting fast and going it alone, as it announced that on May 1 it will leave OPEC, the league of oil-exporting states that since 1960 has played a major role in setting global oil prices. The UAE says that by exiting the cartel it will be able to set its own levels of oil production and no longer be bound by the collective decision-making of OPEC, which has long been accused by critics of placing artificial limits on production in order to boost oil prices. But the decision comes amid mounting …