The respected London-based International Institute for Strategic Studies said President Barack Obama will increasingly have to turn to others for help dealing with the world's problems - in part because he has no alternative.
"Domestically Obama may have campaigned on the theme 'Yes we can'; internationally he may increasingly have to argue 'No we can't,'" the institute said in its annual review of world affairs.
The report said the United States' struggles against insurgent groups in Iraq and Afghanistan had exposed the limits of the country's military muscle, while the near-collapse of the world financial markets sapped the economic base on which that muscle relied.
The report also claimed that the U.S. had lost traction in its efforts to contain Iran's nuclear program and bring peace to the Middle East.
"Clearly the U.S. share of 'global power,' however measured, is in decline," the report said.
The head of another respected London think-tank, Robin Niblett of Chatham House, said the rise in the relative power of China, India, Russia and the European Union has made it harder for the U.S to exercise its influence.
"America should apply changes in leadership style, but I wouldn't overplay the decline because decline is relative," Niblett said. "One should not doubt that the U.S. remains the most powerful nation in the world, but it's difficult to use the power and to use it to influence others."
In addition to a rise in regional powers, Niblett said the U.S. has long been viewed as being part of the problem rather than the solution on many issues, including climate change, the financial crisis, and the failure of the Middle East peace process.
"It's also carrying the baggage of failed policies and of a failed financial approach," Niblett said, referring to the Bush administration. "There's a lot of catching up to be done."
The IISS report praised President Obama, saying that he recognized there was only so much America could do "to impose its views on others."
After years of often thorny relationships between the U.S. and its allies during Bush's administration, President Obama has talked of the need to work with other nations on such issues as the financial meltdown, climate change and nuclear proliferation.
"These are challenges that no single nation, no matter how powerful, can confront alone," Mr. Obama said in April after attending the G20 summit in London.
"The United States must lead the way," he said. "But our best chance to solve these unprecedented problems comes from acting in concert with other nations."
Niblett said many countries "have developed new antibodies to American global leadership. They have built resistance to being told what to do."
The think-tank's report said Mr. Obama could help restore the United States' standing by working with other nations to contain emerging threats to its position as the world's pre-eminent power. Controlling the nuclear ambitions of Iran and North Korea would require help from regional allies, the report said. The same was true of Afghanistan, where the U.S. has had difficulty persuading its NATO partners to follow its lead in boosting the number of troops sent to fight a resurgent Taliban.
"In the next year or two, the greatest demand on U.S. talents and power will be to persuade more to become like-minded and adopt greater burdens," the report said.
Still, Niblett said President Obama was moving in the right direction.
"This administration is far more frank about the U.S. interdependence with rest of the world, and that's a good thing," Niblett said.
By Associated Press writer Raphael G. Satter