Defense officials are wary of China's military growth, and U.S. intelligence agencies have accused Beijing for stealing American high-tech data through computer-based attacks. U.S. officials and security experts are increasingly warning that the United States is highly vulnerable to cyberattacks, including one that could take down the electric grid, financial networks or energy plants.
Republicans say Obama has failed to slow Iran's nuclear program, saying that it could spark an arms race across the Middle East and that it poses the greatest threat to the U.S. and its allies.
Sen. John McCain, the leading Republican on the Senate Armed Services Committee, told his party's national convention that Obama missed an opportunity by not supporting a revolution in Iran.
"For four years, we've drifted away from our proudest traditions of global leadership," said McCain, the GOP presidential nominee in 2008. "We are now being tested by an array of threats that are more complex, more numerous and just as deeply and deadly as I can recall in my lifetime."
Others say that the Obama administration has calmed tensions overseas with Russia, China and other countries that viewed the American invasion of Iraq with suspicion.
"Everyone was afraid that Iraq meant that whenever we thought it was a good idea to bring democracy to a country by force we would do it," said James Lewis, a Washington-based national security expert, now with the Center for Strategic and International Studies. Through diplomatic efforts by the Obama administration, he said that "that level of fear has been tamped down. The global perception of the U.S. is better."
Lewis agreed that Iran may be the one place where the U.S. is no better off than it was four years ago, but he said things are stagnant, not worse. But he blamed the lack of progress on the Iranians and their refusal to engage.
Tensions with China continue over its growing military, as well as cyberactivities and the territorial disputes in the South China Sea. A trip to China this past week by Secretary of State Hillary Rodham Clinton highlighted the friction between Washington and Beijing.
But Clinton and other U.S. officials say that despite the routine disagreements, they can now discuss the issues more freely and frankly with the Chinese, unlike in recent years, when communications were difficult and rare.
"We have strengthened our alliances around the world to protect against future threats, locked down nuclear materials and improved our homeland defenses," said Tommy Vietor, spokesman for the National Security Council. "The U.S. is absolutely safer now than four years ago."