For more than two centuries, great crises have created the opportunity to forge great presidencies.

If the long-simmering Civil War had not broken out a month after his inauguration in 1861, Abraham Lincoln would not be known as the man who saved the Union from dissolution. If Franklin D. Roosevelt had been elected at a time of prosperity, rather than in the midst of the Great Depression in 1932, he would never have overseen the creation of the New Deal—and left a legacy including the Social Security system and minimum standards of living for millions of Americans that we take for granted today.

But simply being elected at a time of crisis is far from a guarantee of success, as Barack Obama well knows.

Obama faces some of the greatest challenges in modern American history as he becomes the 44th President of the United States. As soon as the inaugural balls are over, Obama will enter the Oval Office and have to deal with a number of crises, at home and abroad, as daunting as any since FDR was sworn in.

Like Roosevelt, Obama ran on the theme that the country was broken—and that radical change would be needed to set it back on the right course. George W. Bush leaves office with lower approval ratings than any President since modern polling began—even lower than Richard Nixon in 1974, before he was forced to resign during the Watergate scandal. Today, more than 70 percent of Americans approve of Obama's leadership since his election. But how long the goodwill lasts depends on how quickly, and how well, he handles a wide range of crises.

Job 1: The Economy

In fact, those crises began to swirl around Obama even as he was assembling his Cabinet and preparing to take office. Unemployment is rising—nowhere close to the 25 percent that crippled the country during the Depression, but in the first year of Obama's presidency, nearly 10 percent of the workforce could be jobless. In November alone, a record 533,000 people lost their jobs.

The American car industry is in a state of collapse (click here for related article). Washington has scrambled to save two of Detroit's Big Three—General Motors and Chrysler—from bankruptcy, fearing that their collapse could deliver a knockout punch to an economy that is already reeling. Detroit's woes came on the heels of this fall's financial industry bailout, in which the federal government spent more than a trillion dollars rescuing banks, propping up Wall Street investment firms, and trying to slow the home foreclosures at the root of the turmoil.

Ironically, finding a pathway out of Iraq, which last year looked to be one of the biggest challenges facing the new President, may turn out to be one of Obama's less urgent tasks. In November, President Bush signed an agreement with the Iraqi government that requires most American forces to leave the country by the end of 2010, halfway through Obama's term. During the campaign, Obama promised to devote new troops and attention to Afghanistan, where the situation has been worsening—and many of his advisers fear that just as the U.S. emerges from one military quagmire, it may get stuck in another.

Dangers Abroad

Other parts of the world present equally difficult challenges: November's horrific terrorist attack against Mumbai, India's glittering commercial center, raised the prospect of a confrontation between India and Pakistan, both nuclear powers, and the specter of new terrorist attacks around the globe.

There's a looming confrontation with Iran over its apparent efforts, in defiance of the United Nations, to develop a nuclear weapons program—a capability it may have as soon as this year. Russia is becoming more aggressive, first with its incursion into Georgia last summer, then with its declaration that it would match and defeat any anti-missile systems the United States puts in Eastern Europe. And now China, a bright spot in American relations, is suddenly reeling from the global economic downturn. It has shuttered many of its toy factories, slowed its electronics production, and its economic pain is beginning to spread around the globe.

In short, Obama has no time to settle in. His Chief of Staff, Rahm Emanuel, argued after the election that there could be a silver lining in all of this: The troubles are so great that the nation and the world are ready for big, bold solutions.

"Never allow a crisis to go to waste,'' Emanuel said. "They are opportunities to do big things.''

Such opportunities don't last for long, however. All new Presidents have a "honeymoon'' period, when the public, and even lawmakers from the opposing party, are inclined to give a new Commander in Chief the benefit of the doubt.

Experience & Change

But all it takes is one big mistake to pierce the early enthusiasm—something President John F. Kennedy learned in 1961 when he approved the Bay of Pigs operation, a botched effort to topple the Cuban government of Fidel Castro. Obama's aides don't want to make similar mistakes.

The question now is whether Obama can get enough big changes started, while the country and the world are still captivated by the story of the first black President, and before the political capital he built up during the election is spent.

"Obama's problem is that he's trying to redesign the airplane while he's still flying it,'' says David Rothkopf, a historian and former Commerce Department official. "That's not an easy thing to do, since the No. 1 priority is preventing a crash."

Since Obama ran on a platform of changing the way Washington works, many of his supporters had hoped that he would break from the Bush Administration by bringing progressives and liberals into the government. But his early appointments have been decidedly centrist and include many veterans of the Clinton administration, most notably Senator Hillary Rodham Clinton as Secretary of State. In another move that values experience and continuity over change, Obama is sticking with President Bush's Defense Secretary, Robert Gates, rather than changing the leadership at the Pentagon in the middle of two wars.

"I've sought leaders who could offer both sound judgment and fresh thinking, both a depth of experience and a wealth of bold new ideas," Obama said recently of his choices.

Already there are hints that Obama will delay some of the promises he made during the campaign, including raising taxes on people making more than $250,000 a year and renegotiating trade agreements with other nations in an effort to make them more favorable to American workers. Even on Iraq, he has been less certain on how fast U.S. troops will be pulled out.

It's not unusual for new Presidents to scale back on campaign promises, and there is general agreement that raising taxes in the middle of a recession could be disastrous. But some of Obama's supporters have been restive. David Corn, the Washington editor of Mother Jones, a left-leaning magazine, wrote recently that progressives are "disappointed, irritated, or fit to be tied.''

First Hundred Days

But they have largely stayed quiet, hoping that Obama's strategy is to change the Washington establishment from within.

It's too early to know if he will succeed. But he is preparing a raft of legislation for the first hundred days of his presidency, including a massive economic stimulus package and an overhaul of the nation's health-care system.

Obama's reading of history, he has said, suggests that it is better to do too much than too little: After all, Lincoln's predecessor, James Buchanan, felt overwhelmed and dithered as seven Southern states seceded between Lincoln's election and his inauguration. Herbert Hoover, considered among the smartest men ever to occupy the White House, moved far too slowly after the 1929 stock market crash, and the result was the Depression.

Yet before his inauguration, Obama has had to walk a careful line as far as his involvement in the economic crisis. He has said "we only have one President at a time," a way of reminding people that President Bush is in charge until 12:01 p.m. on January 20. But his aides have worked furiously behind the scenes in the two-and-a-half months between Election Day and Inauguration Day to shape events ahead of the formal transfer of power.

The economic situation Obama inherits is daunting. That loss of more than half a million jobs in November was the biggest drop in 34 years. Stock markets are down about 40 percent from their highs in 2007. From New York to San Francisco and in many places in between, neighborhoods are rife with stories of parents who have lost their jobs, families that have lost their homes, and college savings accounts that have largely evaporated. And teenagers looking for after-school jobs are facing a tough labor market: Part-time and summer hiring are often the first to go in a recession.

If the economy were the only problem facing Obama, it would be tough enough. But he will have to make some critical foreign-policy decisions, starting with Iran, in his first months in office. He has pledged to negotiate with the Iranians directly, something President Bush refused to do. But what if those negotiations fail? Would he use force to stop the Iranians from getting a nuclear weapon?

"In our political system, you can't be a great leader unless you have a great crisis because our constitutional system is oriented against strong leadership in normal times," says Stephen Wayne, a professor of government at Georgetown Univesity. "He's got the crisis, so now we'll see."

In a year's time, Obama will have a harder time blaming problems on his predecessor. If Iraq begins to disintegrate, if Iran threatens military action, if the Taliban surges in Afghanistan, if Pakistan or General Motors collapses—these will be Obama's problems, not legacies of George W. Bush. These will be Obama's moments for greatness, or for failure.