The “surge” in Iraq was in its first weeks, and it seemed hard to imagine that by the time the next president took office, in 710 days, there would be a consensus about the pace of an American withdrawal. The two Palestinian factions, Hamas and Fatah, were talking about a peaceful power-sharing agreement.

The Dow was at 12,580, on the way to 14,000 that summer. General Motors was making money selling cars even while reporting some concerns about “nonprime mortgages” held by its financing division. And the greatest worries about China and India were that their economies were growing so fast they could overheat.

The challenges that Mr. Obama will begin to confront on Tuesday afternoon, in short, bear only a passing resemblance to those on the table on the day two years ago when he conceded that “there is a certain presumptuousness in this — a certain audacity — to this announcement.”

The agenda he is setting out to enact now is significantly altered from what he had in mind then, partly by choice but mostly by circumstance. Over the past two years, and especially in the two and a half months since his election, he has spoken less and less about Iraq and more and more about stabilizing the world economy. Behind the scenes, his national staff has raced to reassess strategies for Afghanistan, Gaza, Iran and Pakistan, even before logging on to their secure computers in the West Wing.

“He’s facing the classic problem of having to handle a number of crises before he’s really got time to set out a long-term architecture,” G. John Ikenberry, a Princeton professor who co-wrote a detailed study of the national security agenda for whoever became the next president. Former Secretary of State Madeleine K. Albright recently called Mr. Obama’s task analogous to “redesigning the airplane while you’re flying it.”

But the shifting reality has done more than force a change in focus. It also led Mr. Obama to re-examine his assumptions about a range of issues, hone his thinking and reach out to new advisers, some of them drawn from Senator Hillary Rodham Clinton’s presidential campaign, some of his aides said.

When it comes to national security in particular, some of his aides see a subtle but distinct shift over the last several years, opening him to the influence of advisers who sound significantly more hawkish than he did two years ago.

So while the world has changed, Mr. Obama has changed with it. But how much?

Aides to Mr. Obama since his arrival in the Senate say his views have not changed as much as some liberal supporters and commentators contend. From the day in 2002 when he stated opposition to the Iraq war, he has said he is not against all wars. And on some issues, including that of striking at terrorism targets in Pakistan, he has sometimes been to the right of both Democratic and Republican rivals.

Yet while he might have been less ideological all along than his initial campaign positioning suggested, his emphasis since the election has been on pragmatism. It is particularly striking that he has signaled, without saying so, that his breaks with the Bush administration will not be as complete as many liberals are hoping. In response to Vice President Dick Cheney’s admonition not to turn his campaign rhetoric into policy until he has taken office and learned “precisely what it is we did and how we did it,” Mr. Obama told ABC News last week that it was “pretty good advice.”

At home, Mr. Obama is confronting an economic and financial crisis that is sure to demand evolving thinking and some trial and error. He has already increased the size of his economic stimulus package. With some of the nation’s big banks under intense strain, he will immediately face tough decisions about redesigning the strategy even as more money is being poured in.

Two years ago, Mr. Obama’s views on Iraq dominated the headlines as he began his campaign by emphasizing his differences with Mrs. Clinton.

That day in Springfield, Mr. Obama animated his supporters with talk about how Iraq was the wrong war. “It’s time to admit that no amount of American lives can resolve the political disagreement that lies at the heart of someone else’s civil war,” he said.

Then, it would have been hard to imagine that in less than two years he would ask President Bush’s defense secretary, Robert M. Gates, to stay on, along with the White House “war czar” for Iraq and Afghanistan. Or that he would choose a secretary of state who portrayed herself as more hawkish than himself and a national security adviser who is a former Marine commandant.

In March, as it was becoming increasingly evident that he would prevail over Mrs. Clinton for the Democratic nomination, Mr. Obama began talking about Iran as the nation that “poses the greatest challenge to American interests in the Middle East in a generation,” and he vowed a few months later never to let the country obtain a nuclear weapon.

His openness to a broad range of viewpoints on national security has become more pronounced with each daily presidential briefing and with deeper dives into National Intelligence Estimates, which his staff says he reads with some skepticism. That is not only because of what the intelligence agencies got wrong in the run-up to the war in Iraq.

“He approaches the intelligence reports the same way he approaches a lot of the things he reads, whether it’s a story in The New York Times or a report from the ground,” said Denis McDonough, a longtime foreign policy aide who is often charged with finding answers to questions Mr. Obama raises. He contends that those who think Mr. Obama has drifted toward more hawkish views were not listening to what he said during the campaign about Iran or Pakistan or Hamas.

One official who is widely reported to be headed for a senior position in the administration and would not talk on the record until his appointment was confirmed said: “Everyone focused on his willingness to engage the Iranians in direct talks, and that was the right thing to do. But they don’t listen to the part that says that if the Iranians don’t come to the table, he’s prepared to talk about cutting off their gasoline and squeezing them on sanctions.”

As his inauguration has approached, Mr. Obama renewed his pledge to engage directly with the Iranians, something Mr. Bush permitted only at the end of his presidency and only at a low government level. Mrs. Clinton, who once cast Mr. Obama’s calls for high-level engagement as an example of his inexperience, will now be in charge of the effort.

But the enterprise is bound to be complicated by the fact that Mr. Bush is handing off to his successor an expanded, covert effort to undermine the Iranian nuclear program, one of many secret programs that Mr. Obama has been briefed about recently and will soon have the opportunity to reauthorize, modify or terminate.

Mr. Obama made clear on Saturday, just before boarding a train in Philadelphia that was supposed to evoke Lincoln’s sweep into Washington in 1861, that he was ready to make good on his promise to increase troop levels in Afghanistan. “Two wars,” he said, “one that needs to be ended responsibly, one that needs to be waged wisely.”

His views on Afghanistan are likely to be heavily influenced by Gen. James L. Jones, his national security adviser, who wrote an influential report a year ago making the case that American and NATO forces were not winning the war and needed a revamped strategy to confront Taliban forces coming over the border from Pakistan. General Jones, in turn, has asked Lt. Gen. Douglas E. Lute, the war czar for Iraq and Afghanistan, to stay on.

General Lute has prescribed a much greater focus on Pakistan and the sanctuary areas along the border, a place Mr. Obama has said he would be willing to send military forces if that is the only way to hunt down members of Al Qaeda and other insurgents.

No one knows where the first crisis of Mr. Obama’s term will erupt. It could be in North Korea, which declared on Saturday that it had “weaponized” much of its nuclear fuel and threatened South Korea. More likely it will be the battle in Gaza that will confront Mr. Obama on Tuesday afternoon. It appears likely that he will be greeted by an Israeli willingness to declare a cease-fire, one that appears partly intended to ease Mr. Obama’s Inauguration Day. No one on his team expects that to last for long.