On inauguration day, a new U.S. president is a demigod, the embodiment of aspirations as vast as they are varied. Over the course of the years that follow, the president inevitably fails to fulfill those lofty hopes. So the cycle begins anew, and Americans look to the next occupant of the Oval Office to undo his predecessor’s mistakes and usher in an era of lasting peace and sustained prosperity.

This time around, expectations are, if anything, loftier than usual. The youthful and charismatic Sen. Barack Obama casts himself as the standard-bearer of those keenest to fix Washington, redeem America and save the world. “Yes, we can,” Obama’s anthem proclaims, inviting supporters to complete the thought by inserting their own fondest desire. Yes, we can: bring peace to the Middle East; reverse global warming; win the global war on terrorism.

Yet Sen. John McCain’s campaign has been hardly shy about fostering grandiose expectations. Speaking earlier this month, while most Americans were fretting about the cost of oil, McCain uncorked one of his patented straight-talking promises: “I’m going to lead our nation to energy independence.” As far as McCain would have us believe, you can take that to the bank.

Will the next president actually bring about Big Change? Don’t get your hopes up.

Regardless of who wins Nov. 4, we should temper our expectations of what George W. Bush’s successor will accomplish, especially on foreign policy.

In reality, presidents don’t make policy; administrations do. To judge by the cadre of advisors they’ve recruited, neither candidate holds much affinity for outside-the-box thinkers. Obama’s “national security working group,” for example, consists chiefly of Democratic war horses, including former secretaries of State Madeleine Albright and Warren Christopher and former national security advisor Anthony Lake -- a group that is not young, not charismatic and not known for innovative thinking.

McCain’s national security team features a strong neoconservative presence, including pundits such as Max Boot and Robert Kagan, along with hawkish Washington insiders such as Randy Scheunemann and James Woolsey. All figured prominently among advocates of invading Iraq; none has yet to repent. Agents of change? Not likely, unless having a go at Iran qualifies as creative thinking.

The very structure of American politics imposes its own constraints. For all the clout that presidents have accrued since World War II, their prerogatives remain limited. A President McCain will almost certainly face a Congress controlled by a Democratic and therefore obstreperous majority. A President Obama, even if his own party runs the Senate and House, won’t enjoy all that much more latitude, especially when it comes to three areas in which the dead hand of the past weighs most heavily: defense policy, energy policy and the Arab-Israeli peace process. The military-industrial complex will inhibit efforts to curb the Pentagon’s penchant for waste. Detroit and Big Oil will conspire to prolong the age of gas guzzling. And the Israel lobby will oppose attempts to chart a new course in the Middle East. If the past provides any indication, advocates of the status quo will mount a tenacious defense.

Then there is the growing gap between American power and the demands of exercising global leadership.

The limits of American power are most obviously apparent in the realm of military affairs. For McCain, Iraq remains the central front in the war on terrorism, and he’ll stay as long as it takes to win. Obama’s central front is Afghanistan, and he wants to bolster the U.S. commitment there. Their disagreement masks a more fundamental problem: The next commander in chief will inherit an intractable troop shortage. The United States today finds itself with too much war and too few warriors. That alone will constrain a president conducting two ongoing conflicts.

A looming crisis of debt and dependency will similarly tie the president’s hands. Bluntly, the United States has for too long lived beyond its means. With Americans importing more than 60% of the oil they consume, the negative trade balance now about $800 billion annually, the federal deficit at record levels and the national debt approaching $10 trillion, the United States faces an urgent requirement to curb its profligate tendencies. Spending less (and saving more) implies settling for less. Yet among the campaign themes promoted by McCain and Obama alike, calls for national belt-tightening are muted.

Above all, there is this: The rest of the world doesn’t take its marching orders from Washington and won’t, no matter who happens to be president next year. Governments will respond to American advice, threats or blandishments precisely to the extent that doing so serves their interests, and no further. This alone sharply restricts what Bush’s successor will be able to accomplish, whether dealing with allies such as Israel and Pakistan or with adversaries such as Iran and North Korea.

Will the tone and tenor of American diplomacy under either a President Obama or a President McCain differ from what we have seen over the last seven years? Yes, and probably in ways that most nations -- and many Americans -- will welcome. But no matter how much charisma or straight talk emanates from the White House, the world will remain stubbornly intractable.

In matters of substance, Big Change will remain elusive. The next president will leave his own imprint on U.S. policy. It just won’t be nearly as distinctive or dramatic as the most enthusiastic Obama and McCain supporters have talked themselves into expecting.

Andrew J. Bacevich, a professor of history and international relations at Boston University, is the author of the new book “The Limits of Power: The End of American Exceptionalism.”