The world greeted the Barack Obama era on Tuesday with excited hopes for the future and deep caution over the difficulties ahead that lay ahead for the first black American president.
While there has been a massive public outpouring of faith in the young leader promising change, his counterparts in other countries have sought to douse expectations.
German Chancellor Angela Merkel wished Obama "the best of luck" but gave the latest of a series of warnings to the new US president this time about sending troops to Afghanistan and dealing with Iran.
She said Obama would not sway Germany to add further to its force in Afghanistan. "We took our decisions based on our capabilities, our skills not on who is president," she told German public television.
Germany has already agreed to increase its Afghanistan contingent to 4,500 soldiers.
Merkel also said she understood Obama's thinking in seeking direct talks with Iran which many countries accuse of seeking nuclear weapons but she was sceptical as to his chances of success.
The German leader has already made it clear to Obama that Europe would not accept what it considers unfair aid to the US auto industry during the financial turmoil.
Obama inherits an economy in crisis, wars in Afghanistan and Iraq, and a conflict in the Middle East where the United States has a key role even though its standing is considered lower than any time for decades.
In Gaza, where a ceasefire barely held after a three week Israel-Hamas war in which more than 1,300 Palestinians were killed, widowed mother of six Leila Khalil said: "Obama won't bring my husband back to life."
"He was martyred and left me with six children to feed on my own. And Obama won't repair our house that was damaged in the raids."
On the other side, Israeli opposition leader Benjamin Netanyahu said: "I took away the impression that Barack Obama understood our distress very well, as well as the cruelty of the enemies we face."
French Foreign Minister Bernard Kouchner said he "shared the admiration and emotion of the whole world" for Obama.
But he added: "I think we should not expect him to immediately solve all America's problems, nor ours. Barack Obama does not have a magic wand."
"He will have a huge job, even if his merits are huge."
Russian Prime Minister Vladimir Putin has also poured cold water on the hopes of optimists.
"I am deeply convinced that the biggest disappointments are born out of big expectations," he said during a trip to Berlin at the weekend.
"Mr Obama looks like a sincere, open person and this of course is appealing," he said.
"That there is such enthusiasm in European countries apparently means that there is big disappointment about what Europeans have seen in US foreign policy recently."
China said Obama would have to work on stronger military ties between the two countries. Ministry of Defence spokesman Colonel Hu Changming said there were "difficulties" in military relations with the world superpower.
"In this new period we hope that both China and the US could make joint efforts to create favourable conditions and improve and promote military-to-military relations," Hu told reporters in Beijing.
Polish Foreign Minister Radoslaw Sikorski told the country's Gazeta Wyborcza his country would back Obama "wherever our interests come together and where our political and economic ability allows."
The paper headlined its article: "Good Luck, Mr President".
International polls have shown huge public support for the Democratic president however.
A BBC World Service poll of people in 17 countries found that most an average of two-thirds believe Obama will improve the relationship between the United States and the rest of the world.
Ghanaians are most positive, on 87 per cent, followed by Italy (79 percent), Germany and Spain (78 per cent each), and France (76 per cent), followed by Mexico and Nigeria (74 per cent each).
Few words of encouragement were given to President George W Bush as he left office, his legacy scarred by the Iraq war.
The Times in South Africa said in a commentary: "The world will say goodbye to a man who has been described as one of America's worst presidents."