Barack Obama was advised early in his presidency to focus on just one thing at a time and postpone health care for another year. The advice was pressed on him not just from outside observers but from trusted aides inside the White House. He rejected it—and for that reason, among others, the health reform that's been delayed for a century, and which he will sign into law this fall, will carry his indelible stamp.

Afterward, though, there will be no period of rest for this president, no time to rein in the scope of his activism. By necessity, he will confront a succession of fateful issues that will not wait. After health reform comes the deluge.

At home, Obama will have to push for a thorough overhaul of financial regulation, overcoming vested interests intent on a return to unfettered business as usual. He carried the case to Wall Street this week, on the anniversary of Lehman Brothers' collapse. Change, he argued, is essential to prevent another speculative boom and bust. He would prefer to shape it cooperatively, and he may have better luck with business than with the Republican Party of "no." But the U.S. Chamber of Commerce is already mounting the barricades; before the battle is over, a confrontational Obama may have to reprise FDR's denunciation of "the forces of greed and privilege."

This president will encounter similar resistance on energy and climate change, issues on which inaction is not an option. To do nothing or too little would endanger not just the environment but the economy for decades to come. It would also renew America's isolation in the world after a brief springtime when the president has reached out to repair our international standing.

The House has already passed a bill; the Senate, with filibustering Republicans determined to see Obama fail at every turn, is the roadblock. The president will have to negotiate with coal state senators in his own party. The process will be as contentious and perilous as the passage of the stimulus and health reform. The tea-baggers will brew a new pot of disinformation and imminent doom. To prevail, Obama will have to draw on a rising perception of economic recovery to strengthen his authority and credibility. But the course he's already navigated as president suggests that when all the shouting and deal-making have ended, he will win most of what he wants.

Add to all this the imperative of far-reaching but difficult reforms to shore up the financial stability of Social Security and Medicare—and it is clear that the coming months, and the years to follow, will not be a quiet time in Congress or the country. And that doesn't even count student loan reform, fiercely opposed by banks and middlemen profiteers; or new Obama nominations to the Supreme Court, which are not only very likely but nearly certain to be fiercely contested; or the expiration of the Bush tax cuts for the wealthy and an end to the ban on gays in the military, each of which will be assailed.

Nor does the president have space to meet a daunting array of domestic challenges in a tranquil world. Take the nuclear threat from North Korea: having agreed to one-on-one talks, Obama predictably has been attacked by critics who apparently prefer a military response—which would be countered by an attack on Seoul, the disruption or devastation of the world's 15th-largest economy, and possibly another global economic tailspin.

On the other side of Asia, the administration has to respond to the specter of a nuclear Iran, with both China and Russia resisting tougher sanctions. Relations with each of them are frayed, as a resurgent Russia seeks hegemony across its borders and China seeks control of natural resources in Africa (while making threats—idle ones, most likely—to stop buying the American bonds that finance our deficit).

The president can't decide to deal with the problems sequentially; they may, and in some cases certainly will, have to be dealt with simultaneously. And all the while, urgent decisions await on Iraq and Afghanistan. In Iraq, there are premature claims of victory in a war Obama opposed; no one knows for sure what will follow an American drawdown and withdrawal. Meantime, we seem to be losing to a resurgent Taliban in Afghanistan. Fifty-seven percent of Americans oppose the conflict there, according to the latest CNN poll. Disillusion with the war was deepened by the Afghan government's replication of Iran's ballot-stuffing, fraud-riddled election. How can we condemn the one and countenance the other?

Sen. Dianne Feinstein, the chair of the Intelligence Committee and no dove, now wants a time limit on our Afghan intervention. The conservative columnist George Will just wants to end it. The generals want more troops—now.

Obama may have to settle for half a loaf or even just a slice: Taliban and warlord control in return for denying sanctuary to al Qaida. The prospect of making Afghanistan a modern, tolerant democracy by force of arms was always bleak, if not impossible. The president will require all his powers of persuasion to justify and sustain a major troop escalation there—or to disengage if and when it's time to admit a qualified defeat. Amid the inevitable charges of being "soft on terrorism," he would have to take every possible measure to prevent another attack on American soil.

Joe Biden predicted that Obama would be tested by crisis in his first six months. In reality, he will be tested over and over again, at home and abroad. The array of challenges he faces is breathtaking. Not since JFK and LBJ, from 1962 to 1965, has a president had to dare and master so much. That period saw a faulting economy reinvigorated by Kennedy's new economics; it saw the civil-rights revolution and the passage of Medicare.

Abroad, there was the Cuban Missile Crisis—which occurred at the same time as a war, all but forgotten now, between India and China that could have enveloped the subcontinent. Then came the Nuclear Test Ban Treaty. And then, of course, there was the downward spiral of Vietnam. Senate Majority leader Mike Mansfield later reported that JFK was prepared to stand down there—but only after the 1964 election. He never had the chance.

Kennedy and Johnson in the end got most of it right—except for Vietnam. That proved to be a fatal exception that closed off an era of progress and left America torn by divisions that took decades to heal.

For Obama, the lessons are clear. We live in a restless, unforgiving time. His predecessor's cataclysmic mistakes abroad and destructive incompetence at home have afforded Obama few options. To succeed, he must be relentlessly activist. As his own instincts tell him, he has no choice but to respond to "the fierce urgency of now."