Dear Mitt Romney,

Congratulations on Florida. Now that you are again the front-runner, and your campaign focus is returning to President Obama, I’d like to call attention to a line you have used repeatedly: “This is a president who fundamentally believes that this next century is the post-American century.” I leave it to the president to describe what he believes, but as the author of the book “The Post-American World,” let me make sure you know what exactly you are attacking.

“This is a book not about the decline of America but rather about the rise of everyone else,” I note at the very outset. I am optimistic about America, convinced that it can prosper in this new world and remain the most powerful country on the planet. But I argue that the age of American unipolarity — which began with the collapse of the Soviet Union — has ended. For a quarter-century after the collapse of communism, the United States dominated the world with no real political or economic competitors. Its ideas and its model — the Washington consensus — became received wisdom everywhere.

Today we are in a different era. In 1990, China represented 2 percent of global gross domestic product. It has quadrupled, to 8 percent, and is rising. By most estimates, China’s economy will become the world’s largest between 2016 and 2018.

This is not simply an economic story. China’s military capacity and reach are expanding. Since 2008 Chinese naval fleets have escorted more than 4,300 ships through the Gulf of Aden. Beijing’s defense spending is likely to surpass America’s by 2025. For its foreign policy activism, look on any continent: A gleaming new African Union headquarters was unveiled in Addis Ababa, Ethi­o­pia, last week. The $200 million-plus complex was financed by China and inaugurated by a high-ranking Politburo member, who arrived with a check for $94 million.

It is not just China that is rising. Emerging powers on every continent have achieved political stability and economic growth and are becoming active on the global stage. Twenty years ago Turkey was a fragile democracy, dominated by its army, that had a weak economy constantly in need of Western bailouts. Today, Turkey has a trillion-dollar economy that grew 6.6 percent last year. Since April 2009, Turkey has created 3.4 million jobs — more than the European Union, Russia and South Africa put together. That might explain Prime Minister Recep Tayyip Erdogan’s confidence and his country’s energetic foreign policy.

Look in this hemisphere: In 1990, Brazil was emerging from decades of dictatorship and was wracked by inflation rates that reached 3,000 percent. Its president was impeached in 1992. Today, the country is a stable democracy, steadily growing with foreign-exchange reserves of $350 billion. Its foreign policy has become extremely active. President Dilma Rousseff is in Cuba this week, “marking Brazil’s highest-profile bid to transform its growing economic might into diplomatic leadership in Latin America,” the Wall Street Journal reported Wednesday. Brazil’s state development bank is financing a $680 million rehabilitation of Cuba’s port at Mariel.

For three decades, India was unable to get any Western country to accept its status as a nuclear power. But as its economy boomed and Asia became the new cockpit of global affairs, the mood shifted. Over the past five years the United States, France, Britain and others have made a massive exception for New Delhi’s nuclear program and have assiduously courted India as a new ally. I could go on.

This is a new world, very different from the America-centric one we got used to over the last generation. Obama has succeeded in preserving and even enhancing U.S. influence in this world precisely because he has recognized these new forces at work. He has traveled to the emerging nations and spoken admiringly of their rise. He replaced the old Western club and made the Group of 20 the central decision-making forum for global economic affairs. By emphasizing multilateral organizations, alliance structures and international legitimacy, he got results. It was Chinese and Russian cooperation that produced tougher sanctions against Iran. It was the Arab League’s formal request last year that made Western intervention in Libya uncontroversial.

By and large, you have ridiculed this approach to foreign policy, arguing that you would instead expand the military, act unilaterally and talk unapologetically. That might appeal to Republican primary voters, but chest-thumping triumphalism won’t help you secure America’s interests or ideals in a world populated by powerful new players.

You can call this new century whatever you like, but it won’t change reality. After all, just because we call it the World Series doesn’t make it one.

comments@fareedzakaria.com